Change handling of netconf cluster transactions

Change handling of netconf cluster transactions

Own actor is created on master for every transaction.
Proxy transactions use this actor for mount point access.
Creation of proxy transaction initiates sending of message
to the master node actor. Master node actor starts
transaciton actor if needed.
Both master and slaves communicate with device via
proxy transactions.

For write transaction, submit and cancel cause actor stop.
There can be single instance of read actor, which can be
reused for all transactions.

Change-Id: Iea39d794e8f2026b76f049409a718b6d94f41649
Signed-off-by: Andrej Mak <andrej.mak@pantheon.tech>
(cherry picked from commit 446738525190ad92736f1a8868e3263737276aff)