This solves a problem that caused a hang on shutdown related to
the lifetime of the ping timer completion handlers used in WebSockets.
* Turn the ping timer back on
* Use std::weak_ptr for WebSockets timer callbacks.
* Disable WebSocket pings if frequency in the .cfg is non-positive.
* Deprecate rpc_admin_allow section from configuration file
* New port-specific setting 'admin':
* Comma-separated list of IP addresses that are allowed administrative
privileges (subject to username & password authentication if configured)
* 127.0.0.1 is no longer a default admin IP.
* 0.0.0.0 may be specified to indicate "any IP" but cannot be combined
with other IP addresses.
