Add retry mechanism to get snapshot method

During snapshot finalization the snapshot file is getting overwritten. If we try to read the snapshot file at this moment we can get back an empty or incomplete snapshot. This change adds a retry mechanism in case of such failure.
[TESTS] Specify unicast hosts and transport port range specifically
The p/c queries must always be wrapped in a CustomQueryWrappingFilter in fquery parser.
Also made assertions more descriptive when failing.
[Test] Strengthen SimpleIndicesWarmerTests.testEagerLoading.
Disabled query size estimation in percolator, because this is too expensive cpu wise.
Lucene's RamUsageEstimator.sizeOf(Object) is to expensive.
Query size estimation will be enabled when a cheaper way of query size estimation can be found.

Closes #5372
Relates to #5339
Significant_terms aggregation identifies terms that are significant rather than merely popular in a set.
Significance is related to the changes in document frequency observed between everyday use in the corpus and
frequency observed in the result set. The asciidocs include extensive details on the applications of this feature.

Closes #5146
upgrade randomized-testing to 2.1.1

Note that the standard `atLeast` implementation has now Integer.MAX_VALUE as upper bound, thus it behaves differently from what we expect in our tests, as we never expect the upper bound to be that high.
Added our own `atLeast` to `AbstractRandomizedTest` so that it has the expected behaviour with a reasonable upper bound.
See https://github.com/carrotsearch/randomizedtesting/issues/131
[Doc] Add a chart about the relative error of the percentiles aggregation.
[TEST] pre check download service working

Seen during CI tests, it could appears that the download service is not available for any reason.

This fix in test will check before each test which requires an internet access (annotated with @Network) that the download service we are testing is still working.

It won't fail the test but will mark the test as `Ignored` in case of failure.
Allow scripts to return more than 4 values in aggregations.

A missing call to ArrayUtil.grow prevented the array that stores the values
from growing in case the number of values returned by the script was higher
than the original size of the array.

Close #5414
Allow to share multicast socket within jvm
Due to bugs in jvm (specifically OSX), running zen discovery tests causes for "socket close" failure on receive on multicast socket, and under some jvm versions, even crashes. This happens because of the creation of multiple multicast sockets within the same VM. In practice, in our tests, we use the same settings, so we can share the same multicast socket across multiple channels.
This change creates an abstraction called MulticastChannel, that can be shared, with ref counting. Today, the shared option is only enabled under OSX.
closes #5410
[TEST] Make sure that "assertNoFailures(searchResponse);" is used instead of "assertThat(searchResponse.getFailedShards(), equalTo(0));" to see what is failing.
Removed redundant "assertThat(searchResponse.getFailedShards(), equalTo(0));" checks
[TEST] Made sure that the parent exists (`foo` is the parent type and not `parent`)
Support externalValue() in mappers

Some mappers do not support externalValue() to be set. So plugin developers can't use it while building their own mappers.

Support added in this PR for:

* `BinaryFieldMapper`
* `BooleanFieldMapper`
* `GeoPointFieldMapper`
* `GeoShapeFieldMapper`

Closes #4986.
Relative to #4154.
Add _cat/plugins endpoint

If we want to have a full picture of versions running in a cluster, we need to add a `_cat/plugins` endpoint.

Response could look like:

```sh
% curl es2:9200/_cat/plugins?v
node component                        version   type url                                   desc
es1  mapper-attachments               1.7.0       j                                        Adds the attachment type allowing to parse difference attachment formats
es1  lang-javascript                  1.4.0       j                                        JavaScript plugin allowing to add javascript scripting support
es1  analysis-smartcn                 1.9.0       j                                        Smart Chinese analysis support
es1  marvel                           1.1.0      j/s http://localhost:9200/_plugins/marvel Elasticsearch Management & Monitoring
es1  kopf                             0.5.3       s  http://localhost:9200/_plugins/kopf   kopf - simple web administration tool for ElasticSearch
es2  mapper-attachments               2.0.0.RC1   j                                        Adds the attachment type allowing to parse difference attachment formats
es2  lang-javascript                  2.0.0.RC1   j                                        JavaScript plugin allowing to add javascript scripting support
es2  analysis-smartcn                 2.0.0.RC1   j                                        Smart Chinese analysis support
```

Closes #4824.