BUG-1105: Implement subtree filtering in netconf

Implement subtree filtering according to RFC 6241 section 6.
Current implementation first gathers response XML, then
applies filtering. Since netconf server is pluggable,
modules that might consume significant amount of time
to build their response can read the filter and optimize
their result. Even if modules do not support filtering,
their result is filtered by SubtreeFilter right before
sending out the aggregated response.

Change-Id: Ica04f8f3adfe06599e54a577fceca0508554d667
Signed-off-by: Tomas Olvecky <tolvecky@cisco.com>
Merge "BUG-1105: Implement subtree filtering in netconf"