Truncate last unaligned bytes of access log

Truncating unaligned bytes of access log allows use of access log
even if last bytes are unaligned to blocksize.
Avoid throwing a ShortReadException which just disables the access log
Change-Id: I6e82fb0aa4b62e892e092c98654df71bee3f16d6
Reviewed-on: http://review.couchbase.org/36995
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: buildbot <build@couchbase.com>