Make random number generation be per session in the WiredTiger library,
and per thread in wtperf and test/checkpoint, reference #1223.
Merge pull request #1224 from wiredtiger/random-per-session

Fix shared variable collisions in random number generation.
Fix a hang in LSM compact. Refs #1225
Merge pull request #1226 from wiredtiger/lsm-compact-hang

Fix a hang in LSM compact. Refs #1225