Add bfdmonClient_NotifyReadAndDispatch() function.

Part of the client lib API. Used by a monitor client application to read
incoming notify messages from the connection to the monitor server.

Does not yet parse the messages.
Add bfdSessionCompare() to core.

Removed bfdMonitorSessionIdCompare() from bfd-monitor.c and just use
common session compare function.
Add bfdStateFromStr().

Utility function for converting a string into a bfdState value.
Implement Notify dispatching.

The monitor client library now tracks subscriptions and can notify the
monitor client application about session state changes.
Update TODO file.