Also remove an unnecesssary copy of the closeSilently() method

A persistent multi-version map: open the file in exclusive or shared mode; allow to read and update the file header.

A persistent multi-version map: split based on size (memory), not number of entries; allow to estimate the memory used by unsaved changes

A persistent multi-version map: avoid 'magic' casting in the comparator, because strictly Collections.binarySearch could swap the parameter objects in the call to the compare method. This should also simplify porting the code to C.

(a) any accidental further access to it
(b) throwing an exception if the client code calls close() more than once

Stand-alone command line version of the profiling tool (will be merged with the profiling tool later on).