yum-presto not on by default

[Bill Nottingham]

Jonathan Dieter (jdieter at gmail.com) said: 
> The second one has to do with the fact that when rebuilding the rpms, we
> have to recompress the data, and xz compression is over 10x slower than
> gzip.

Stats of the day...

I took an existing xz-compressed RPM, and built a newer release of that
package with varying xz levels. The numbers are:

- the new package size at that XZ compression level
- the time to create a new deltarpm from a delta to that new version

level 1
1416k

real	0m0.935s
user	0m0.893s
sys	0m0.036s

level 2
952k

real	0m0.839s
user	0m0.790s
sys	0m0.048s

level 3
848k

real	0m2.762s
user	0m2.702s
sys	0m0.055s

level 4
832k

real	0m2.902s
user	0m2.817s
sys	0m0.084s

level 5
824k

real	0m3.269s
user	0m3.131s
sys	0m0.128s

level 6
812k

real	0m4.364s
user	0m4.026s
sys	0m0.164s

level 7 (what we do now)
812k

real	0m4.698s
user	0m4.516s
sys	0m0.176s


So... just set the xz compression level to 2, let it be that way for future
builds, and go about our business?

Bill