Compaction removes old entries from the changes-stream.

With this fix, running memcachetest in a loop...

    ./memcachetest -h 127.0.0.1:11210 -L 2 -l

...shows that after several compactions, db file sizes stay in the
same range rather than blowing up into the gigabytes...

    $ ls -l tmp/00/df/default-bucket/
    total 189896
    -rw-r--r--  1 steveyen  staff  55611965 Apr 25 18:27 0-13.store
    -rw-r--r--  1 steveyen  staff  36518289 Apr 25 18:27 0-14.store
    -rw-r--r--  1 steveyen  staff   3339740 Apr 25 18:27 55104dc76695721d380704bb7b4d7c03_0-13.views
    -rw-r--r--  1 steveyen  staff   1742813 Apr 25 18:27 55104dc76695721d380704bb7b4d7c03_0-13.views.compact
    -rw-------  1 steveyen  staff       157 Apr 25 18:21 settings.json

cbugg: close bug-607
Prefer 0666 when writing settings.json file.

I recall hearing from Dustin Sallings that 0666 is better than 0600.