dnsdist: Protect GnuTLS tickets key rotation with a read-write lock

dnsdist: Protect GnuTLS tickets key rotation with a read-write lock

Otherwise a thread can replace the shared pointer hold by the
GnuTLSIOCtx while another thread is accessing it. The usage count
is not incremented since no copy is made, so the content might get
deleted while a thread is still accessing it, leading to
use-after-free and possibly a crash.