hashsplit.py: simplify code and fix BLOB_MAX handling.

hashsplit.py: simplify code and fix BLOB_MAX handling.

This reduces the number of lines without removing functionality.  I renamed
a few constants to make more sense.

The only functional change is that BLOB_MAX is now an actual maximum instead
of a variable number depending on buf.used().  Previously, it might have
been as large as BLOB_READ_SIZE = 1MB, which is much larger than BLOB_MAX =
16k.  Now BLOB_MAX is actually the max.

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>