Redis: Fixes name of last-save status variable

On our NR, the 'changes since last save' shows up as blank. 
According to http://redis.io/commands/INFO the variable 
to check has a slightly different name.

Also adds a useful metric (time taken to save db) which
can indicate problems if it spikes.
Merge pull request #198 from courtenay/patch-1

Redis: Fixes name of last-save status variable