Increase MAX_BLOCK_SIZE to 8192

The current MAX_BLOCK_SIZE only allows files of up to ~87 MB in size.
Considering that a windows winpe ramdisk can grow to about ~270 MB,
it would just fail after block nr 65535. Increasing this constant
will allow larger files to be transmited.

It is not mandatory for the client to ask for this block size, this
is just a higher cap, up to which a client may negotiate.

Preferably, this would be configurable per backend, and not hardcoded
in session.py.