Docs: Rewrote the scroll/scan docs

Closes #6774
[TEST] more consistent toString on cluster state, print with assertTimeout
Docs: grammatical error

Closes #6716
Docs: YAML mappings use colons, not equals sign

Closes #6750
Revert "[Discovery] immediately start Master|Node fault detection pinging"

In #6706 we change the master validation to start pining immediately after a new master as ellected or a node joined. The idea is to have a quicker response to failures. This does however create a problem if the new master has yet fully processed it's ellection and responds to the ping with a NoLongerMasterException. This causes the source node to remove the current master and ellect another, only to find out it's not a master either and so forth. We are moving this change to the feature/improve_zen branch, where the improvements we made will cause the situation to be handled properly.

This reverts commit ae16956e072bea317ea481f65f2e110dc48fde17.
Core: don't replace indices within the ActionRequest with the concrete ones, and make sure check blocks is executed on concrete indices

Concrete indices is now called multiple times when needed instead of changing what's inside the incoming request with the concrete indices. Ideally we want to keep the original aliases or indices or wildcard expressions in the request.

Also made sure that the check blocks is done against the concrete indices, which wasn't the case for delete index, delete mapping, open index, close index, types exists and indices exists.

Closes #6694
Closes #6777
[Discovery] verify connect when sending a rejoin cluster request

When a master receives a cluster state from another node, it compares the local cluster state with the one it got. If the local one has a higher version, it sends a JoinClusterRequest to the other master to tell it step down. Because our network layer is asymmetric, we need to make sure we're connected before sending.

Closes #6779
Docs: Improved the docs for nested mapping

Closes #1643
Aggregations: Aggregation names can now include dot

Aggregation name are now able to use any character except '[', ']', and '>". Dot syntax may still be used to reference values (e.g. in the order field) but may only defence the value directly beneath the last aggregation in the list. more complex structures will need to be accessed using the aggname[value] syntax

Closes #6702
Docs: Changed the path separator for Windows in the README

Closes #1854
Core: trigger refresh when the live version map is using too much RAM

When refresh_interval is long or disabled, and indexing rate is high,
it's possible for live version map to use non-trivial amounts of RAM.
With this change we now trigger a refresh in such cases to clear the
version map so we don't use unbounded RAM.

Closes #6443
Translog: change flush_threshold_ops to unlimited by default, so we only flush by byte size by default

If you are indexing tiny documents then the previous default (5000)
was too low, causing excessive fsyncs with high indexing rates.  With
this change we now only flush by byte size (200 MB) by default for
better indexing performance for tiny documents.

Closes #6783
Add assertBusy helper test method
We use awaitBusy in our tests, the problem is that we have to check if it awaited or not, and then try and keep around somehow more info around why the predicate failed and a timeout happened.
The idea of assertBusy is to allow to simply write "regular" test code, and if the test code trips, it will busy wait till a timeout. This allows us to keep around the assertion information and properly throw it for information that is inherently kept in the failure itself.
[TEST] move tribe tests to use assertBusy
Add missing pre built analysis components
closes #6770
Remove intern calls on FieldMapper#Names
remove internal callas on FieldMappers#Names, we properly reuse FieldMapper, so there is no need to try and call intern in order to reuse the names. This can be heavy with many fields and continuous mapping parsing.
closes #6747
[API SPEC] Update indicies.stats group param
add more options to many mappings test
[ENGINE] Prevent NPE if engine is closed while version map is checked

We check if the version map needs to be refreshed after we released
the readlock which can cause the the engine being closed before we
read the value from the volatile `indexWriter` field which can cause an
NPE on the indexing thread. This commit also fixes a potential uncaught
exception if the refresh failed due to the engine being already closed.

Relates to #6443
Closes #6786
[Engine] checkVersionMapRefresh shouldn't use indexWriter.getConfig(). We run it out of lock, the indexWriter may be closed..

Relates to #6443, #6786

Closes #6794