Improve logging of persister and netconf client.

Add a debug message measuring time spent waiting for netconf server to reply in NetconfClient.
Log with error level failure to send/recieve netconf message in config-persister-impl.
Add TODOs to config-manager's dependency resolver for detecting cycles.

Change-Id: I412abad141b40eb9346ef404d2efeba8c197e018
Signed-off-by: Tomas Olvecky <tolvecky@cisco.com>