Simply initialize TLS context if DynamicDHGroups change.

We used to do init_keys() if DynamicDHGroups changed after a HUP, so
that the dynamic DH modulus was stored on the disk. Since we are now
doing dynamic DH modulus storing in crypto.c, we can simply initialize
the TLS context and be good with it.

Introduce a new function router_initialize_tls_context() which
initializes the TLS context and use it appropriately.