ACCUMULO-2581 Revisit the WorkAssigner impls to make some proper OO hierarchy.

The SequentialWorkAssigner is nice, and does what it's meant to, but it's
not nearly as fast as can be done with multiple tservers. It's probably a
good idea to keep a sequential and unordered work assigner in good working order
ACCUMULO-378 (re)Create the UnorderedWorkAssigner which doesn't care about replaying files in any order

Increase the default size of the threadpool too.