Shm: Introduce first support for pools

[rbx]

This uses boost::interprocess::adaptive_pool to store reference counts for unmanaged region buffers when these are "softly" copied by FairMQ.

One additional small managed segment per unmanaged region is created to store the reference count pool. Default segment size is 10,000,000 bytes, which can be changed through regionConfig in code. RefCount object size is 2 bytes, so this should be good for up to 5 million counts.

This replaces the old functionality, which would store the ref counts as separate message objects in the main managed segment. Thus, this should reduce fragmentation of the main segment.

I also add an example that uses this, but tests are also already in place, which tested the old implementation.

[dennisklein]

I guess you can squash the third and last commits?