Experimental promises support & Proposals metrics

Implemented enhancements:

• Added --experiment-promise-check flag #464 (soffokl)

Fixed bugs:

• Fixed getting version from tag #467 (soffokl)
• Goimports check do nothing since goimport is not installed for CI #450

Closed issues:

• Payments. Provider stores Consumer promises (state) #347

Merged pull requests:

• Update development environment setup docs #461 (tcharding)
• Force bin/build to run #459 (tcharding)
• Reduce noise when running check\_golint #457 (tcharding)
• Check we have goimports #453 (tcharding)
• Run goimports with builder #452 (tadovas)
• Cherrypick of "Identity check disabled by default, added flag to enable it. #363" #445 (Waldz)
• Added fetchQuality to the proposals endpoint with a fake data #444 (soffokl)
• Added promises validation and storing for the service provider. #347 #404 (soffokl)
• Move towards a more modular connection manager #385 (vkuznecovas)
• Compile in geoip2 database and avoid external file reading #281 (tadovas)

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