block-rbd: Add a bunch of statistics about merging

This allows to find out what's going on and see where/when
requests are issued, to hopefully guide toward better merging
strategies.

Signed-off-by: Sylvain Munaut <s.munaut@whatever-company.com>
block-rbd: Allow configuration of the maximum merge size

Syntax is to use max_merge_size=N option (N being in bytes)

Signed-off-by: Sylvain Munaut <s.munaut@whatever-company.com>
debian: Update the debian build files

Signed-off-by: Sylvain Munaut <s.munaut@whatever-company.com>