[contactsd] Disconnect handlers after use

The handlers for account readiness events are only intended to run
once; disconnect them after use to ensure the handler is not run a
number of times.
[contactsd] Do not discard account change information on delay

Preserve the account change flags if we must delay processing of the
account addition.
Merge pull request #74 from matthewvogt/ready-signal

[contactsd] Disconnect handlers after use