Split sal-resconf-connector module into two modules by RESTCONF version supported

Currently, we support two RESTCONF versions. We support draft-bierman-netconf-restconf-04 (see https://tools.ietf.org/html/draft-bierman-netconf-restconf-04) and also RESTCONF RFC 8040 (see https://tools.ietf.org/html/rfc8040).

Both of these version are implemented in single sal-restconf-connector module. There is a lot of boilerplate code copypasted on multiple places across module. Also, it's not that obvious what parts of the module implement what RESTCONF version.

We should split this single module into two new ones based on the RESTCONF version they implement.