fixed "leverage rev_host grouping" issue

[mzhilyaev]

fixed by using "group by rev_host" query instead of pulling each individual url from places

[Mardak]

Could you run some timing measurement to see if this changes performance much for the better or worse?

[mzhilyaev]

timing for addon loading

1. Profile is being read first time
   info: query time 2614
   info: number crunching time 4
2. Profile is being read again - Firefox is loaded second time in a raw
   info: query time 140
   info: number crunching time 5

It appears that simply reading places.sql is what takes the most time. When the file is buffered, query time is reduced by 20x

[Mardak]

What are the numbers before with individual urls and after with rev_host?

[mzhilyaev]

Disregard the data above. The new data here
Before the rev_host change, the addon timing was this:

info: query completion 1757
info: number crunching 5

After rev_host implementation timing is this:
info: query time 138
info: number crunching time 5

I no longer can reproduce time difference between fresh and cached porfile

[Mardak]

The timing looks good. This pull request has unnecessary changes though. Just take the d1763df commit and rebase it to master and don't include the timing commit.