Fix fstat struct leak.

fstat structs are never reclaimed.  this is bad.  this patch adds a command for the post-merge read file update to use to remove fstats entries, so as to bound the set of files that will be considered for merges and status updates and keep 100 times down.

Testing notes:

to test this, I built a riak devrel with this patch, created a 20GB ramdisk on data/bitcask/ (10 should be enough), and added the following line to riak.conf: 

```
bitcask.max_file_size = 1MB
```

Then I started and attached to the console ran ran `repro:start()` from this module: https://gist.github.com/evanmcc/3fbab9ca05bcfbacc01d

I then set up a shell (all of this was done in 2-3 screen sessions) watching the output of status:

```bash
while :; do ./bin/riak-admin status | grep put_fsm_time; sleep 30; done
```
Over time, without this patch applied, 100 times will increase linearly with the time the test has been running. Depending on whether you've applied the patch from #115, they'll either stay flat (with both patches) or increase linearly but more slowly (just this patch, as empty merge files pile up they slow down new file creation on size wrap).

Usually the difference is plain after a few hours, but I usually leave the tests running overnight.