Tweak forced eviction a little more: don't retry, but give in-memory splits more chance of happening.
Add statistics tracking forced eviction.
Don't block the oldest reader in the system forever.  Try harder to evict when we aren't keeping the oldest ID pinned.

refs #754
Back out a change to __wt_txn_am_oldest: that isn't changing.
Avoid division by zero if the cache size has not yet been set in a shared cache.
Back out another part of the original change: we need to get a snapshot when positioning a cursor, even if it is going to auto-commit.
Merge pull request #755 from wiredtiger/eviction-unblock

Avoid eviction deadlocks
Merge branch 'develop' into fine-grained-durability

Conflicts:
	src/include/wiredtiger.in