Minor rename: __session_add_{btree -> dhandle}.
Improve handling of a pathological case where the sweep and eviction servers are fighting for the handle list lock.  The big improvement is to have the eviction server not hold the handle list lock while walking a file.  In addition, the sweep server can interrupt the eviction server in order to get the handle list lock and discard a file from cache.

refs #1391
Nit: rename a variable.
Merge branch 'develop' into sweep-eviction-improvements

Conflicts:
	src/session/session_dhandle.c
Revert to sweep checking a handle's reference count is zero inside the handle list lock.
Merge pull request #1402 from wiredtiger/sweep-eviction-improvements

Sweep eviction improvements