Introduce a condition variable for application threads waiting for eviction
candidates.

Application eviction used to poll, but it's hard to get the polling
interval right for all cases.
Use a broadcast when signalling a condition variable.  The only case where more than one thread can be waiting is when application threads are waiting for the eviction server, and we want all of them to wake up as soon as there are pages in the LRU queue.
Merge pull request #506 from wiredtiger/evict-cond-wake

Introduce a condition variable for application threads waiting for eviction candidates.