ACCUMULO-3305 Use stopProcessWithTimeout instead of directly using Process API

We don't know if the start of a Process might be delayed, and, if we
call destroy before this happens, then waitFor appears to hang
indefinitely. Use the existing thread pool to guard against
an inf loop.
Merge branch '1.6'
ACCUMULO-3297 allow metadata table reads even if all files have been allocated
ACCUMULO-3297 merge 1.6 to master