[sr-dev] [kamailio/kamailio] Weight-based contacts call termination distribution (#2167)

[Marco Capetta]

### Description
Implement a new TM logic to distribute calls to contacts belonging to the same subscriber similar to the weight mechanism that is used in lcr module. This mechanism should support primary/backup, load-sharing or a combination of both.

### Expected behavior
Currently, with the existing serial forking using q-value:
 - if the q-values are different, the devices will ring serially but always in the same order
 - if the q-values are equals, the devices will ring in parallel

With the new mechanism, instead, I have to be able, for example, to register 2 devices and send:
 - the 50% of the call to device 1 and then to device 2 as a backup
 - the 50% of the call to device 2 and then to device 1 as a backup


### Possible Solutions
A possible approach is to extend the existing load_contact function and re-use the q-value as a weight.
For example:
 - contact 1 has Q value 1
 - contact 2 has Q value 0.5
 - contact 3 has Q value 0.5
 - contact 4 has Q value 0
Then kamailio distributes the calls using the following proportion:
 Q : total_Q = call_prob : 100
So:
 - contact 1 has probability to ring of 50% (1 * 100 / 2)
 - contact 2 has probability to ring of 25% (0.5 * 100 / 2)
 - contact 3 has probability to ring of 25% (0.5 * 100 / 2)
 - contact 4 has probability to ring of 0 (0 * 100 / 2) but it is used as backup option if all other contacts fail

This should solves the limitations of serial forking and allows to achieve equal or predictable weight-based call termination distribution.

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https://github.com/kamailio/kamailio/issues/2167
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