Retry DB transactions on badger.ErrConflict

[edigaryev]

To fix the tests in https://cirrus-ci.com/task/5344327489028096 failing because of #114 (comment).

Also log the causes of HTTP 500 errors.

[edigaryev]

Found the issue: Transaction Conflict. Please retry during POST to /v1/workers.

This is probably caused by a concurrent PUT from an Orchard Worker to /v1/workers/microsun.local.

[CLAassistant]

All committers have signed the CLA.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}