Improve file name matching in log collection.

With old code we would crash if for some reason there's a file named
like debug.log.1asdf in log directory.

Change-Id: I0a32b61b432e2b38aba7ada38fdbfa310f475073
Reviewed-on: http://review.couchbase.org/37891
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
[ale] Make it more clear that we require gzip headers.

Change-Id: I5b18d7b3c20a425edc194634de2ea52b777c04b9
Reviewed-on: http://review.couchbase.org/37892
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>