fix: update cloned msg under lock to avoid races with branch injection

[vladpaiu]

Summary
Aims to fix double free crash happening when a branch gets injected into a transaction which has not relayed yet.

Details

1. process Y creates the transaction and calls t_wait_for_new_branches() ( either directly or via lookup() )
2. procesX tries to inject a new branch and makes a clone of t.uas.request https://github.com/OpenSIPS/opensips/blob/master/modules/tm/t_fwd.c#L1064
3. proces Y calls t_relay() and updates t.uas.request https://github.com/OpenSIPS/opensips/blob/master/modules/tm/tm.c#L1339, frees old add_rm ( along other old fields ) https://github.com/OpenSIPS/opensips/blob/master/modules/tm/sip_msg.c#L1286
4. procesX tries to free it's fake req, https://github.com/OpenSIPS/opensips/blob/master/modules/tm/t_msgbuilder.h#L403 ,finds a different pointer than the one in t.uas.request, ending in a double free

Solution
When doing t_relay(), update the UAS request under lock, to avoid these race conditions

Compatibility
Should be backwards compatible