hashsplit: join buffers more efficiently

hashsplit: join buffers more efficiently

Drop buffer_concat in favor of join_bytes, since we can use b''.join()
to join the two put() chunks in python 3, which should be notably more
efficient than copying the memoryview bytes before joining them.

In python2, also use join when we just have bytes, otherwise
accommodate the fact that you can't join bytes and buffers there, and
you can't append bytes to a buffer.  The case we currently care about
should still end up being handled the same as befoere (in the first
clause), i.e. buf + bytes.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>