Rename block manager statistics with "block manager" prefix so they sort
together.

Add a statistic for block manager memory allocations while holding the
system locked.
Move the cache of WT_EXT structures into the WT_SESSION_IMPL handle so
we (hopefully) never call malloc while holding the block manager's live
lock, and minimize modifying the WT_BLOCK structure's cache lines.

Abstract out private session handle cleanup for the block manager and
reconciliation.
Update the test suite to match the statistics name changes.
Fix spelling
Merge pull request #765 from wiredtiger/session-ext-alloc

Moving the WT_EXT structure cache to the session handle.