[Reviewer: Alex] Add NullSASLogger to disable SAS logging from the PingHandler

Alex,

Please can you review this fix to add a NullSASLogger to disable SAS logging from PingHandler?  This was raised as part of Metaswitch/ralf#88.

I've updated UT (as part of homestead) but not live-tested yet.

Matt
As discussed, there are pros and cons either way, but you're happy with this approach.