FT-304 Add stress test coverage for db->get_fragmentation(db)
FT-304 Fix an oops in the blocktable, first exposed in a MySQl test and
now by src/tests/stress_test7.cc
FT-300 Use an environment variable to determine which file the block
allocator trace gets written to
FT-300 Fix a bunch of issues with the replay tool, including:
- Failure to handle multiple allocators at once (oops)
- Failure to handle a clean shutdown where allocators are destroyed by
  the trace gracefully and so we can't get stats for them after the run
  (need to clean this up eventually)
- Added line numbers to error messages so debugging is easier
FT-300 Add best-fit strategy for replay testing