feat(kv): clear RPC method which completely cleans storage

[rustatian]

Reason for This PR

closes #735

Description of Changes

• Implement Clear method for all supported storages.
• Memcached via flush_all.
• Memory - destroy memory region.
• Redis - flush DB (only connected to the KV DB)
• BoltDB - delete and recreate bucket.

License Acceptance

By submitting this pull request, I confirm that my contribution is made under
the terms of the MIT license.

PR Checklist

[Author TODO: Meet these criteria.]
[Reviewer TODO: Verify that these criteria are met. Request changes if not]

• All commits in this PR are signed (git commit -s).
• The reason for this PR is clearly provided (issue no. or explanation).
• The description of changes is clear and encompassing.
• Any required documentation changes (code and docs) are included in this PR.
• Any user-facing changes are mentioned in CHANGELOG.md.
• All added/changed functionality is tested.

[codecov]

Codecov Report

Merging #736 (60001db) into master (87d023d) will decrease coverage by 0.06%.
The diff coverage is 58.53%.

@@            Coverage Diff             @@
##           master     #736      +/-   ##
==========================================
- Coverage   67.57%   67.51%   -0.07%     
==========================================
  Files          94       94              
  Lines        4824     4885      +61     
==========================================
+ Hits         3260     3298      +38     
- Misses       1151     1165      +14     
- Partials      413      422       +9     

Impacted Files	Coverage Δ
plugins/kv/drivers/memcached/driver.go	49.50% <40.00%> (-0.50%)	⬇️
plugins/redis/kv.go	60.34% <50.00%> (-0.37%)	⬇️
plugins/kv/drivers/boltdb/driver.go	52.46% <52.63%> (+0.01%)	⬆️
plugins/kv/rpc.go	71.23% <57.14%> (-1.50%)	⬇️
plugins/memory/kv.go	62.85% <100.00%> (+2.25%)	⬆️
plugins/websockets/pool/workers_pool.go	62.74% <0.00%> (-7.85%)	⬇️
plugins/websockets/validator/access_validator.go	77.77% <0.00%> (-3.48%)	⬇️
plugins/broadcast/rpc.go	62.50% <0.00%> (+9.37%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ce53a8e...60001db. Read the comment docs.