Add automatic throttling for LSM based on merge speed. Update merge_threads.

The automatic throttling starts if there are too many chunks in an LSM tree
that have never been merged.

Increase the default number of LSM merge threads to 2 from 1. Since one
merge thread can now be pretty much dedicated to creating bloom filters.
Merge branch 'develop' into lsm-merge-throttle
Increase the default value for session_max to 100.  This allocates more space in the connection handle, but otherwise should have minimal impact on performance, because we track the number of sessions in use.

This change was prompted by increasing the default number of LSM merge threads to 2, which means there is an overhead of 3 sessions per LSM tree (2 merge threads and a checkpoint thread).
Merge pull request #819 from wiredtiger/lsm-merge-throttle

Add automatic throttling for LSM based on merge speed. Update merge_threads