Changes

server: Split network code into its own file

Separate code related to network socket handling into its own separate
file, as already done in osmo-pcap-client.
This allows easily finding out which code is operating on the socket and
which code is operating on the pcap file being locally stored.
This will make modifying subsystems easier too.

Change-Id: I41d6e0398605becbea109a7bb5aac8981d65ab1b