Add a handler for managing client side keepalive (#1773)

Motivation:

Clients can be configured to use keepalive which periodically sends
PING frames to the server, typically only if streams are open, but this
behaviour is configurable. Clients should also close connections
after periods of inactivity (if no streams are open).

Modifications:

Add a client channel handler and state machine which handles client
side keepalive and connection idling.

The handler also emits connection events to the next handler, these
events can be used to determine whether to take the connection out of
service (i.e. stop creating new streams on it).

Results:

The client can manage keepalive and connection idling.

Author: glbrntt