dialog: simplified dlg referencing on creation and spiral detection
dialog: release of dialog for not-forwarded initial request done by creator

- avoid releasing new dialog in case of upstream paralel forking
  or retransmission by other processes than the other which created the
  dialog
- it could result in destroying the dialog in initial state if the
  request was not forwarded yet, likely to happen because creator
  process runs more callbacks
dialog: when adding a new dialog, lock the slot until the structure is linked

- search for dialog based on sip attributes and if no result found, then
  lock the hash table slot until the new structure is built and linked
  in the table
- should avoid simulataneous creation for same dialog if there is a
  not-handled retransmission or parallel forking upstream, resulting in
  many processing managing duplicated requests