Scalability fix for Helgrind: reduce the size of ScalarTS (scalar

Scalability fix for Helgrind: reduce the size of ScalarTS (scalar
timestamps) from 16 to 8 bytes.  This halves the size of vector
timestamps and reduces the amount of memory needed to run programs
that have many threads and/or many synchronisation events.

The tradeoff is that Helgrind must abort the run if the program
creates more than 2^20 (1.0e+6) threads or performs more than 2^44
(1.76e+13) synchronisation events.  Neither of these seem like a
significant limitation in practice.  It's easy to argue that a limit
of 2^44 synch events would take at a minimum, several CPU months on a
very fast machine.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@11570