R4R: hide dbKeybase's constructor

[alessio]

crypto/keys.New(string, string) now returns a lazy keybase
instance. Function's signature has changed, this this is
an SDK Go API breaking change.

Remove client.MockKeyBase, superseded by
crypto/keys.NewInMemory()

Closes: #3592

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.
• Wrote tests
• Updated relevant documentation (docs/)
• Added entries in PENDING.md with issue #
• rereviewed Files changed in the github PR explorer

---

For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[cwgoes]

Doesn't build at the moment.

[alexanderbez]

Changes LGTM apart from one minor change. Also CI is failing.

[codecov]

Codecov Report

Merging #3593 into develop will decrease coverage by 0.05%.
The diff coverage is 13.04%.

@@             Coverage Diff             @@
##           develop    #3593      +/-   ##
===========================================
- Coverage    61.47%   61.42%   -0.06%     
===========================================
  Files          186      186              
  Lines        13978    13978              
===========================================
- Hits          8593     8586       -7     
- Misses        4838     4844       +6     
- Partials       547      548       +1

[jackzampolin]

LGTM 👍 way to kill the old keybase 🗡