Since libspatialindex is not thread safe on all platforms, and
have expressed desire to remove the thread safety that they DO
have on remaining platforms, it's safer and easier for us
to manually add locks to QgsSpatialIndex and be gauranteed that
this class is thread safe on all platforms and libspatialindex
versions.
Also improve docs for the class.
Since the underlying libspatialindex is not thread safe
on some platforms (e.g. Windows), manual calls to detach()
must be made if a QgsSpatialIndex is to be accessed across
multiple threads.
Note that for platforms on which libspatialindex is thread
safe, calling detach() has no effect and does not force the
deep copy.
