Upgrade to Milton 2.6

Appears to be working. Solves the problem where proxied vector reads cause the entire
file to be written to local disk. Instead only the actual data that is to be returned
to the client is buffered. If this is more than 100 KB, that data is buffered on local
disk. The file is deleted after successful return of the data.

There are still problems though: In case of failures reading the data from the pool,
the temporary file is not deleted. The entire file is still downloaded from the pool
to the door (even when only the requested data is buffered), and the multi-range writer
in Milton is extremely inefficient as it processes one byte at a time (it fails to
override the multi-byte write methods).

The new version supports using cookies to remeber when a client has already logged in.
I disabled this feature for now.

Target: trunk
Request: 2.11
Request: 2.10
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Require-notes: yes
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: https://rb.dcache.org/r/7530/