Iterating over all entries of a std::map is rather slow, so add a
interval map to manage the free entries.

A first attempt to use boost::icl::interval_set for this was abandoned;
while the releaseId() function would be just 1 line, GCC 4.8.2 at least
is unhappy about boost icl headers for non-obvious reasons:

UnpackedTarball/boost/boost/icl/discrete_interval.hpp:45:225: error:
 default argument for template parameter for class enclosing ‘void
 boost::icl::boost_concept_check_dummy45(boost_concept_check45*)’

Change-Id: I7b767aefee57df7743dc13a694b6a61abdd536c7