feat(memdb): add memdb package

[peakji]

No description provided.

[codecov-commenter]

Codecov Report

Base: 94.38% // Head: 91.39% // Decreases project coverage by -2.99% ⚠️

Coverage data is based on head (e017b95) compared to base (07f9d8e).
Patch coverage: 82.87% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #18      +/-   ##
==========================================
- Coverage   94.38%   91.39%   -3.00%     
==========================================
  Files          27       33       +6     
  Lines        1745     2358     +613     
==========================================
+ Hits         1647     2155     +508     
- Misses         69      139      +70     
- Partials       29       64      +35     

Impacted Files	Coverage Δ
internal/engine/memdb/queue.go	68.83% <68.83%> (ø)
internal/engine/memdb/task.go	74.28% <74.28%> (ø)
internal/engine/memdb/promise.go	76.19% <76.19%> (ø)
internal/engine/memdb/topic.go	83.33% <83.33%> (ø)
internal/engine/memdb/memdb.go	97.56% <97.56%> (ø)
internal/engine/memdb/indexer.go	100.00% <100.00%> (ø)

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[peakji]

TODO: benchmark count performance.
Reference: hashicorp/go-memdb#83