If there are only two LSM worker threads, don't let the cache get full.

Allow the first thread to do flushes as well as switches and drops
if there are only two threads.

Refs #1441, but this is a hang seen from that test/format configuration,
not a segfault.
Tidy up previous change. Remove dead code.
Fix new test for LSM workers max.
Merge pull request #1447 from wiredtiger/lsm-two-workers

If there are only two LSM worker threads, don't let the cache get full.