Feature/add wal utilization check

[Matze-S]

Added a check to monitor wal file utilization in a given past period of time.

The check is based on the modification time of the WAL files and is therefore imprecise within the scope of the WAL segment size, but should be sufficient to monitor storage consumption.

[hasegeli]

I ran the tests, and got a failure:

t/02_wal_amount.t .......... 6/12 DBD::Pg::st execute failed: server closed the connection unexpectedly
	This probably means the server terminated abnormally
	before or while processing the request. at t/02_wal_amount.t line 76.
# Looks like your test exited with 255 just after 8.
t/02_wal_amount.t .......... Dubious, test returned 255 (wstat 65280, 0xff00)

[Matze-S]

Hi, I have also run all the tests in a fork of the travis-CI and all tests that are working without the change are fine. The tests with the newer perl version and the newest postgres database also failed before. I will try to reproduce the failure and fix it - sorry.