osmo-mgw: refactor endpoint and trunk handling

The trunk and endpoint handling in osmo-mgw is still very complex and
implemented in various places (mostly mgcp_protocol.c). Also we use
still integers for endpoint identification, which is not flexible enough
to address timeslots/subslots on an E1 trunk. Some refactoring is needed.

  - get rid of integers as endpoint identifiers, use strings instead and
    find the endpoint based on its string name on the trunk.

  - identify the trunk based on the trunk prefix given in the endpoint
    name.

  - refactor trunk and endpoint allocation. Aggregate functionality in
    in mgcp_endp.c and mgcp_trunk.c. Also remove non-reusable code that
    relates to the still exisiting, but unfinished E1 trunk support.

  - refactor rate counters, put them into a separate module and do no
    longer allocate them per trunk. Allocate them globally instead.

Change-Id: Ia8cf4d6caf05a4e13f1f507dc68cbabb7e6239aa
Related: OS#2659
diff --git a/include/osmocom/mgcp/Makefile.am b/include/osmocom/mgcp/Makefile.am
index 036b4ca..fb7654f 100644
--- a/include/osmocom/mgcp/Makefile.am
+++ b/include/osmocom/mgcp/Makefile.am
@@ -7,5 +7,7 @@
 	mgcp_sdp.h \
 	mgcp_codec.h \
 	mgcp_ctrl.h \
+	mgcp_trunk.h \
 	debug.h \
+	mgcp_ratectr.h \
 	$(NULL)