Allow generation of Big Requests extension

Allow the generation of Big Requests extension as part of core.
Generate Big Requests extension
Implement querying of maximum request length

Provide a method for user to query the maximum request length, which is
either the field returned by connection setup, or the reply from Big
Requests extension.
Send big requests if available

Allow sending of big requests if the extension is available. Big
requests will only be sent if the size is larger than normal, indicated
by the 'maximumRequestLength' field of connection setup.