Add is_empty to Engine API

[tabokie]

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[codecov]

Codecov Report

Merging #228 (d1b4634) into master (5e8e4bb) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #228      +/-   ##
==========================================
+ Coverage   97.13%   97.14%   +0.01%     
==========================================
  Files          29       29              
  Lines        9030     9066      +36     
==========================================
+ Hits         8771     8807      +36     
  Misses        259      259              

Impacted Files	Coverage Δ
src/engine.rs	97.39% <100.00%> (+0.05%)	⬆️
src/memtable.rs	99.17% <100.00%> (+<0.01%)	⬆️
src/consistency.rs	91.11% <0.00%> (-2.23%)	⬇️
src/env/default.rs	92.57% <0.00%> (+0.57%)	⬆️

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