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