Allow the start offset for test_brw be based on the thread number. This

Allow the start offset for test_brw be based on the thread number.  This
allows the threads to be writing to different parts of the file (instead
of all overwriting the same part of the file), to better simulate a real
client load.

Make thread "offsets" start at zero for objids and file offsets (we need
to keep the actual thread numbers > 0, because thread 0 is the master).

Make be_verbose 64-bit clean, and also handle very large counts.

Validate LOV default stripe count <= OST count.