ACCUMULO-2575 Fix concurrent modification in GCWAL.

Didn't use the iterator to remove entries in the for-loop.
ACCUMULO-2575 Incorrect conditionals caused an NPE.

Stubbed a test for implementing later
ACCUMULO-2575 First end-to-end test to make sure the GC closes WALs

The GC should, when the WAL is no longer in use, start marking WALs
as being closed in the replication table. The test attempts to make
sure that this is properly happening by observing Status records moving
from open to closed.