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
imported patch spelling
Fix compiler warning:
../src/log/log.c: In function '__wt_log_scan':
../src/log/log.c:696: warning: 'start_lsn.unused' may be used uninitialized in this function

GCC is getting paranoid.
Only consider eviction "busy" if there are updates in progress.

refs #754
Retry forced eviction even if the oldest ID hasn't moved on.  An in-memory split may succeed.

This reverts part of b4c8021f0affee34f1d60e54e87a35fc7fe0cafa.