Make Releasable extend AutoCloseable.

Java7's AutoCloseable allows to manage resources more nicely using
try-with-resources statements. Since the semantics of our Releasable interface
are very close to a Closeable, let's switch to it.

Close #5689
Improved SearchContext.addReleasable.

For resources that have their life time effectively defined by the search
context they are attached to, it is convenient to use the search context to
schedule the release of such resources.

This commit changes aggregations to use this mechanism and also introduces
a `Lifetime` object that can be used to define how long the object should
live:
 - COLLECTION: if the object only needs to live during collection time and is
   what SearchContext.addReleasable would have chosen before this change
   (used for p/c queries),
 - SEARCH_PHASE for resources that only need to live during the current search
   phase (DFS, QUERY or FETCH),
 - SEARCH_CONTEXT for resources that need to live until the context is
   destroyed.

Aggregators are currently registed with SEARCH_CONTEXT. The reason is that when
using the DFS_QUERY_THEN_FETCH search type, they are allocated during the DFS
phase but only used during the QUERY phase. However we should fix it in order
to only allocate them during the QUERY phase and use SEARCH_PHASE as a life
time.

Close #5703
Upgrade to Lucene 4.7.2
Check for no open issues before build release
Prevent fsync from creating 0-byte files

This is related to LUCENE-5570 where fsync creates a 0-byte file
if the file does not exists. This commit adds the patched lucene
version using Java 7 APIs as well as a note to replace this method
with the upcomeing IOUtils#fsync in Lucene 4.8

This commit cleans up FsImmutableBlobContainer#writeBlob to make
use of Java7 Auto-Closing features and ensures that the directory
the blob was written to is fsynced as well if possible.
Remove unused FileSystemUtils#copyFile
[BUILD] remove leftover print statement
Separate persistent and global metadata serialization settings
Benchmark documentation

Moving benchmark documentation under the search section.

Closes #5786
Fix snapshot status with empty repository

The snapshot status command with empty repository should return current status of currently running snapshots in all repositories.

Fixes #5790
Separate benchmark API endpoints

Separates benchmark API endpoints into separate files according to API
funtionality. This makes it easier for our tests and clients.

Closes #5787