[rrl-datasrc] datasrc extensions to support RRL.

there are two major extensions:
- extend DataSourceClient::FindResult so it includes the number of labels
  of zone's origin
- add getMatchLabelCount() virtual method to ZoneFinder::Context.

These are generally intended to be used to identify the "query name"
for the purpose of RRL efficiently.
[rrl-datasrc] corrected a merge error: match label cnt should be 0 for NOTFOUND.

also, removed some comments that are not applicable anymore.
[rrl-datasrc] added some more comments
[master] Merge branch 'rrl-datasrc'

Resolved conflicts in:
	src/lib/datasrc/memory/zone_table.cc