BGP: fix locking order error on dynamic protocol spawn

We missed that the protocol spawner violates the prescribed
locking order. When the rtable level is locked, no new protocol can be
started, thus we need to:

* create the protocol from a clean mainloop context
* in protocol start hook, take the socket

Testsuite: cf-bgp-autopeer
Fixes: #136

Thanks to Job Snijders <job@fastly.com> for reporting:
https://trubka.network.cz/pipermail/bird-users/2024-December/017980.html