Prepared geometry is not thread-safe

[dbaston]

The current prepared geometry implementations are not thread safe. In dbaston@c196055 I updated the various lazy initialization methods of PreparedPolygon to use std::call_once. Running a multi-threaded test under valgrind --tool=helgrind shows problems in MCIndexSegmentSetMutualIntersector. There is a lot of state manipulation this class that would need to be removed, while remaining within (or updating) the SegmentSetMutualIntersector interface.

[dbaston]

PR: #826