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