Add a mutex to IndexSet and use it to make IndexSet::do_compress() thread-safe.

Add a mutex to IndexSet and use it to make IndexSet::do_compress() thread-safe.

IndexSet has a number of 'mutable' member variables. The only function that modifies them is
'do_compress', which is called by 'compress', which is in turn called by all of the
'const' member functions. To make all of these 'const' member functions thread-safe, we
need to use a mutex in 'do_compress'. This patch does this.

I have verified that no other 'const' function actually modifies any of the 'mutable'
member variables, so only guarding 'do_compress' by the mutex is sufficient.