core: new parameter - max_branches

- the maximum number of uac branches can be set via config
- default value is 12 (old static value for MAX_BRNACHES)
- the upper limit is 31, it has to be at least 1
- example:
  max_branches=16
tm: transaction uses number of branches specified by max_branches global parameter

- the number of UACs per transaction is based on max_branches global
  parameter