Changes

WIP: ipaccess: Conver BSC OML & RSL link to use stream_srv

This in turn allows running BSC Abis interfaces through io-uring
backend, which should provide performance improvements when used.

TODO: Re-introduce the sign.delay Tx timer to fix issues with ancient
nanoBTS.

TODO: Get rid of ipaccess_fd_cb() in osmo-bsc.git ipaccess-config by
implementing its own e1i_line ipaccess driver there (due to many
peculiarities: tcp/ipa cli instead of srv, different OML port).

Related: SYS#7063
Related: OS#5755
Related: OS#5756
Change-Id: Idf241c8f2fdb86d090d4132a9b316b7236402232