Split poet into registration and worker services

[poszu]

Closes #355
Closes #373
Closes #380

Refactored Service by splitting it into 2 entities:

• Registration
• Worker (still named Service in the code)

And two interfaces that they communicate with:

• WorkerService
• RegistrationService

Registration

It's the front-end service that the node talks to. It is responsible for:

• scheduling poet rounds according to round config
• registering new challenges
• persisting rounds' database
• persisting proofs' database
• serving proofs

When a time comes, it atomically closes the open round, builds a membership tree, and opens a new round. The root of the membership tree is passed to the worker service via the WorkerService interface.

Registration asynchronously waits for proofs coming from the Worker and stores them in the DB.

Worker

It's the backend responsible for executing rounds scheduled by Registration. It registers at the RegistrationService interface for closed rounds and listens for rounds to execute on the returned channel. When a round is finished, it passes the proof back to the RegistrationService.

inMemory transport

It's an entity implementing both WorkerService and RegistrationService to connect the Registration with the Worker. It's very simple and uses 2 channels to pass data between them.

[codecov]

Codecov Report

Merging #364 (5d92ff8) into develop (b651e0d) will decrease coverage by 0.1%.
Report is 10 commits behind head on develop.
The diff coverage is 78.2%.

@@            Coverage Diff            @@
##           develop    #364     +/-   ##
=========================================
- Coverage     73.7%   73.6%   -0.1%     
=========================================
  Files           19      23      +4     
  Lines         1833    1954    +121     
=========================================
+ Hits          1351    1439     +88     
- Misses         358     388     +30     
- Partials       124     127      +3     

Files Changed	Coverage Δ
registration/pow_verifier.go	96.1% <ø> (ø)
shared/shared.go	71.5% <ø> (-4.0%)	⬇️
db/migrate.go	64.0% <25.0%> (ø)
registration/db.go	54.2% <54.2%> (ø)
state/state.go	60.8% <60.8%> (ø)
server/config.go	67.6% <64.8%> (ø)
registration/registration.go	75.8% <75.8%> (ø)
server/server.go	79.6% <76.5%> (+3.7%)	⬆️
rpc/rpcserver.go	85.1% <78.2%> (+6.3%)	⬆️
service/service.go	75.6% <78.3%> (-1.1%)	⬇️
... and 7 more

... and 1 file with indirect coverage changes