Rejecting invalid fields inside of completion field

Only valid fields (input, output, weight and payload) are now allowed inside a completion field.
This makes sure that typos like "inputs" are caught on indexing.
Improve exception handling for river startup

Due to a change in 0.90 some exceptions may show up when starting a river,
even though this is not a problem as the shard has not been initialized
yet and a retry is scheduled.
Remove @Required annotation

The @Required annotation is used for documentation purposes only anyway.
Add FieldMappers before adding Mappers (used for parsing) and while holding the relevant's ObjectMapper mutex

Adding the mappers first could cause the wrong FieldMappers to be used during object parsing

Also:
Removed MergeContext.newFieldMappers, MergeContext.newObjectMappers and relatives as they are not used anymore.
Similarly removed MergeContext.addFieldDataChange & fieldMapperChanges as they were not used.

Closes #3844
Update Lucene to version 4.5.0.
plugin -remove deletes bin directory
I tried to install a plugin directly from disk, but used the wrong arguments. Totally my fault for not RTFM properly.

However, by following the instructions printed out by `bin/plugin`, I ended up deleting my $ES/bin directory.

```
$ pwd
/tmp/elasticsearch-0.90.5
$ ls
LICENSE.txt    NOTICE.txt     README.textile bin            config         lib
$ bin/plugin --install file:///tmp/foo.zip
-> Installing file:///tmp/foo.zip...
Failed to install file:///tmp/foo.zip, reason: plugin directory /tmp/elasticsearch-0.90.5/plugins already exists. To update the plugin, uninstall it first using -remove file:///tmp/foo.zip command
$ bin/plugin -remove file:///tmp/foo.zip
-> Removing file:///tmp/foo.zip
Removed file:///tmp/foo.zip
$ ls
LICENSE.txt    NOTICE.txt     README.textile config         lib
```

I reproduced the problem in 0.90.5 and the latest master.
Closes #3847.
plugin -remove deletes bin directory
Adding test for #3847.
Improved warm-up of new segments.

* Merged segments are now warmed-up at the end of the merge operation instead
  of _refresh, so that _refresh doesn't pay the price for the warm-up of merged
  segments, which is often higher than flushed segments because of their size.
* Even when no _warmer is registered, some basic warm-up of the segments is
  performed: norms, doc values (_version). This should help a bit people who
  forget to register warmers.
* Eager loading support for the parent id cache and field data: when one
  can't predict what terms will be present in the index, it is tempting to use
  a match_all query in a warmer, but in that case, query execution might not be
  much faster than field data loading so having a warmer that only loads field
  data without running a query can be useful.

Closes #3819
Remove extra 'processors' anchor in threadpool docs
completely terrible way to do this right now
add RamAccountingTermsEnum