[Serusers] Manipulate from_uri

[Richard]

> Is avpops is something that could be used already in production? Using
> external routing script made me lazy to check new features in ser :)
> 

We use it extensively in production. You can probably wait for new functions
coming out before considering switch from external routing script in order
to get the performance gain. The particular functions I'd like to have are
avp value manipulation and loading any sip header into an avp. Also lots of
existing functions will probably be enhanced to take advantage of avps, e.g.
prefix() and strip().

Richard