Tweak the LSM throttle and merge algorithms.  Take into account the average
time a chunk stays active when deciding how aggressively to merge.

refs #676
Fix a signed / unsigned warning.
Fix a bug where a read-uncommitted scan was keeping the oldest transaction ID
in the system pinned.

This happens to be what LSM does during a merge.  Since LSM now also uses the
oldest transaction ID to determine when it is safe to flush an in-memory chunk,
the net result was that merges were blocking data being flushed.

refs #676