Cellular Network Infrastructure

As was seen in #4061, we're not doing a good job in closing connections if they're dead. Let's improve this situation by implementing an IPA PING sender / PONG receiver which closes+reconnects (client) or simply closes (server) the respective connection.

I recently added ipa_keepalive_fsm_start to libosmo-abis. It's used only in osmo-remsim,
but should probably be used by virtually any of our programs that implement IPA
multiplex, starting from BTS (Abis), BSC (Abis), MSC (SCCPlite, GSUP), HLR (GSUP),
all our CTRL interfaces, ... - but anything beyond RSL+OML in BTS+BSC is out of scope for this
ticket. I'll add separate tickets about this.

Whether or not that specific FSM is used: The IPA PING/PONG logic should exist everywhere.