feat(tx-builder): get actual gas price from node

[davidyuk]

closes #1786

This PR is supported by the Æternity Foundation

To protect from funds loss in case of a bug/misusage, I propose to add an upper limit for the gas price substituted by default. Known issues:

A single active client would compete with himself

If somebody posts transactions from a single client with high blockchain utilization, the gas price will increase for no reason. This is a special use case, I'm adding a note to batch-requests.md.

Overall solution doesn't protect from mining time fluctuation

With a low utilization, it is expected to get a transaction mined within 3 seconds. Currently, it is cheap to increase utilization at any moment. The current solution would reflect utilization change in a minute. So, a user may experience arbitrary changes of mining time.

I've wrote a script to create several clients pushing transactions one by one and to visualize that gathered data. On the screenshot, I've added about 5 dynamic price clients and removed all of them with an interval from the middle to the end. It looks +/- as expected.

[codecov]

Codecov Report

Attention: Patch coverage is 90.16393% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 80.18%. Comparing base (ad859ae) to head (09d19bf).
Report is 1 commits behind head on develop.

Files	Patch %	Lines
src/tx/builder/field-types/gas-price.ts	84.61%	2 Missing and 2 partials ⚠️
src/tx/builder/field-types/fee.ts	91.30%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1884      +/-   ##
===========================================
+ Coverage    80.07%   80.18%   +0.11%     
===========================================
  Files          103      103              
  Lines         3468     3503      +35     
  Branches       738      750      +12     
===========================================
+ Hits          2777     2809      +32     
- Misses         383      384       +1     
- Partials       308      310       +2     

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[dincho]

Better than static. The test included in this PR would be very helpful in future too.