crypto/keyring: deprecate lazy keybase

[alessio]

Remove redundant lazy keybase implementation altogether.

Created LegacyKeybase interface to restrict capabilities to
only those required by keys commands that deal with
legacy keybase such as update and migrate.

Rename keyring.New() -> keyring.NewLegacy().

Rename client/keys.NewKeyBaseFromDir -> NewLegacyKeyBaseFromDir.

crypto/keyiring.NewInMemory() now returns a in-memory keyring.
BackendMemory is added yet not exposed via command line
--keyring-backend flag. keys add uses it when --dry-run flag
is on.

Remove the Update method from the Keybase interface.

Code cleanup, removed tons of duplicate test cases.

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For contributor use:

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

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For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
• Reviewers assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #5889 into master will increase coverage by 0.00%.
The diff coverage is 47.91%.

@@           Coverage Diff            @@
##           master    #5889    +/-   ##
========================================
  Coverage   56.77%   56.78%            
========================================
  Files         344      342     -2     
  Lines       20307    20047   -260     
========================================
- Hits        11529    11383   -146     
+ Misses       7896     7815    -81     
+ Partials      882      849    -33     

[alexanderbez]

ACK

[fedekunze]

ACK. Plz remove extra files