CBSE-1444: added logging diagnostics to /diag

This commit also implements /diag/ale endpoints that returns json
representation of logging state.

Change-Id: I4192252ed2218a0e8b1bbc70bb90b234c39b5a37
Reviewed-on: http://review.couchbase.org/42216
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>