Changes

library: add generic Mutex API for parallel components

In certain scenarios, it's required to ensure that only one of multiple
parallel components executes a specific code block at any given time.

This, for example, is the case for the S1GW testsuite, where we want to
simulate multiple eNBs establishing E-RABs.  Each E-RAB triggers the
IUT (osmo-s1gw) to send a PFCP Session Establishment Request, and there
is currently no way to correlate which session belongs to which eNB.
This problem can be solved by ensuring that only one eNB is triggering
the PFCP Session Establishment Request(s) at a time.

This patch implements a generic Mutex API, which can also be used
by other testsuites that orchestrate multiple parallel components.

Change-Id: Id71f43bd5fc78d4bb4417d6c01fcff8112ea6032