Bug 2834: Fix Restconf POST to wait for completion

This changeset introduces a blocking mechanism for Restconf POST to ensure that
the transaction completes before a response is returned, as is consistent with
Restconf PUT behavior.  checkedGet() is utilized to ensure write transaction
completion.

Change-Id: I74f68b883f6037ec62644a468d3cbb9370d8d25f
Signed-off-by: Ryan Goulding <ryandgoulding@gmail.com>