Database: rename 'update_generation' to 'touch' and simplify the signature

The 'update_generation' function was defined as
- always bump the generation count (in contract to 'update')
- also transform one binding to another value

In practice all users were using it only to bump the generation count
like "touch"ing a file. Therefore we remove the transform ability and
rename the function "touch".

Signed-off-by: David Scott <dave.scott@eu.citrix.com>