SCHEMA-128. Add minimum size option to KijiTablePool.

The option to require a minimum table pool size is added in this commit.
Kiji will aggressively open the min number of connections the first time a
table connection is requested and will maintain at least that size of a pool
indefinitely.

mvn clean verify passes with the new unit test that checks that the pool gets
initialized to the correct size when the first get() is called.

@sqrl can you have a look at this?