refactor: Make db stuff internal/private

[AndrewSisley]

Part of #200

Makes everything in the db package that doesn't need to be public (db.NewDb, Errors, and the Options) private/internal. Had to do a bit of rejigging in the bench package, so watch out for that (results same as develop branch).

godocs now looks like this (godoc -http=:6060 => http://localhost:6060/pkg/github.com/sourcenetwork/defradb/db/):

[codecov]

Codecov Report

Merging #291 (ba0f343) into develop (764be70) will increase coverage by 1.38%.
The diff coverage is 70.66%.

@@             Coverage Diff             @@
##           develop     #291      +/-   ##
===========================================
+ Coverage    63.30%   64.69%   +1.38%     
===========================================
  Files           83       81       -2     
  Lines         9230     9035     -195     
===========================================
+ Hits          5843     5845       +2     
+ Misses        2771     2574     -197     
  Partials       616      616              

Impacted Files	Coverage Δ
db/collection_delete.go	58.13% <ø> (ø)
db/collection_update.go	42.10% <50.00%> (ø)
db/db.go	53.84% <64.28%> (+0.80%)	⬆️
db/query.go	43.39% <66.66%> (ø)
db/schema.go	34.14% <66.66%> (ø)
db/collection.go	53.43% <78.94%> (ø)
db/collection_get.go	51.02% <100.00%> (ø)
db/sequence.go	65.11% <100.00%> (ø)
api/http/api.go

[orpheuslummis]

LGTM. Although I suggest adding a comment to the Option type along the lines of

Option is a function that, when applied, configures a field on db

[AndrewSisley]

LGTM. Although I suggest adding a comment to the Option type along the lines of

Option is a function that, when applied, configures a field on db

That would be inconsistent with the rest of our public members 😁 - jokes aside though - I think this pattern is pretty common in golang and would not be worth noting atm. Would perhaps be better to leave it as-is until we properly document it with the rest of public members?

[orpheuslummis]

I think this pattern is pretty common in golang and would not be worth noting atm

Sounds preferable not to over-document idiomatic patterns indeed.

[jsimnz]

LGTM.