Bug 31033: Explicitly define max child processes for SIP server

Bug 31033: Explicitly define max child processes for SIP server

By default, the SIP server appears to only use 1 child process for
responding to SIP connections.

This change makes this explicit in the configuration, which should
make it so that people who need more than 1 simultaneous SIP connection
can know to just increase the value for the "max_servers" parameter
in the SIPconfig.xml file.

Test plan:
1. Add "max_servers='1'" to your SIP configuration file
2. koha-sip --restart kohadev
3. Open 3 terminals
4. Run "telnet localhost 6001" on 2 terminals
5. On the 3rd terminal, run the following:
ss -l -n -t
ps -efww | grep "sip"
6. Note that there are 2 processes called
kohadev-koha-sip: perl /kohadevbox/koha/C4/SIP/SIPServer.pm /etc/koha/sites/kohadev/SIPconfig.xml
One of these processes is the parent of the other
7. The Recv-Q in the "ss" output should show 1
(This means that 1 of your telnet connections is in the server's TCP backlog)
8. Celebrate as the configuration works as expected

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>