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 a block size as large as this. It
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.