RFC: improve dpkg-scanpackages performance with cached md5sums

[Michael Burian <michael.burian@sbg.at>]

Problem:

Creating Packages.gz with dpkg-scanpackages takes lot's of time for 
large repositories.

The main reasons why it is so slow is that all checksums of all 
packages, even those that did not change from the previous run, are 
recalculated every time.

Solution:

I've extended dpkg-scanpackages to accept a "--md5cache" | "-5" command 
line option that enables caching and reusing of md5sums.

When not used one ends up with stock dpkg-scanpackages behavior where 
all checksum are recalculated every time. Else md5sums of scanned 
packages are cached on the first run and reused on successive runs.

With cached md5sums, the time to create Packages.gz for my private 
repository (~600MB) dropped from over 1 minute to about 7 seconds on a 
PII/400Mhz.

Would it make sense to include such a feature into official
dpkg-scanpackages?