Move /groups from master process

[neilisfragile]

No reason for it to be on master, should help CPU-bound homeservers running in worker mode.

[richvdh]

this applies to groups apis other than /groups itself; /joined_groups may be a larger culprit

[neilisfragile]

/joined_groups, /publicised_groups, /groups/.../profile, /groups/.../rooms

[richvdh]

fixed by #6866 I guess