Skip rate limit for login, capability negotiation

Disable message rate limit during nick registration and capability
negotiation (Network::Initializing).  Once network initialization
is complete (Network::Initialized), re-apply configured rate limits.

Helps fix connection timeouts during login on servers with SASL
enabled, multiple capabilities negotiated, and a strict 10s login
time.

Add flag 'forceUnlimited' to updateRateLimiting() which temporarily
overrides user preferences to skip all rate limiting.  User
preferences can be applied afterwards by calling updateRateLimiting()
without setting 'forceUnlimited'.

Resolves GH-236.