BUG-8618: add pause/unpause mechanics for tell-based protocol

BUG-8618: add pause/unpause mechanics for tell-based protocol

When we are transitioning to/from paused state, we need to remove
all frontend-related state, including pending transactions, to ensure
ShardDataTree does not track them.

When we change to unpaused leader, we can reconstruct the state
from the journal -- the rest will be forwarded from the frontend anyway.

Change-Id: I28d486d1a6695e21dd7e6518609680d54e5a15eb
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>