pass context and respect cancellation when calling signer server

[ronnno]

signing on blocks should be done under a context object, signer service must respect context cancellation.

additionally, we could implement defensive timeouts on LH side - not trusting signer service to respect context cancellation. @gadcl what do you think? (the question of defensive timeouts is relevant to all inter-service calls but is made especially urgent in the case of signer service because it is actually remote)