fix math of calculating scaled price for marketplace

[etam]

1. revert from msg.price / task.header.max_price back to task.header.max_price / msg.price
2. put this calculation, with handling of 0, into a function in marketplace module.

Related issues: #3453, #3479, #3964

[Krigpl]

What kind of complications are we talking about? It fails with an error? There's test_rust.py which is supposed to test proper integration with Rust and I'd expect a test there that guarantees no "complications in rust part".

[etam]

The complication was not because of python-rust communication of float('inf') value, but because of calculations done in order_providers_impl (for which I expanded tests in marketplace.py file).
The problem is when alpha = 0, because inf * 0 = nan. And using inf would totally break cases when 0 < alpha < 1, because the score would be always inf.
By using float max it's at least giving a good impression of behaving correctly. I tried to talk with @mbenke about it, but he's not responding right now.

[codecov]

Codecov Report

Merging #3976 into b0.19 will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##            b0.19    #3976      +/-   ##
==========================================
+ Coverage   89.18%   89.19%   +<.01%     
==========================================
  Files         205      205              
  Lines       18315    18320       +5     
==========================================
+ Hits        16335    16341       +6     
+ Misses       1980     1979       -1