[HFX-1022] CA-118027: Don't send RPC calls that are too large

The request size limit is known by xapi and so slaves shouldn't try and send
requests that it knows will be rejected.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>

Conflicts:
	ocaml/database/master_connection.ml

Resolved-by: Si Beaumont <simon.beaumont@citrix.com>