Add database adapter pkg

[kanataxa]

Refs: #29

Add database_adapter package for supporting other databases.

[codecov-io]

Codecov Report

Merging #30 into master will decrease coverage by 0.00%.
The diff coverage is 88.57%.

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
- Coverage   71.55%   71.54%   -0.01%     
==========================================
  Files          21       21              
  Lines        8772     8776       +4     
==========================================
+ Hits         6277     6279       +2     
- Misses       1770     1771       +1     
- Partials      725      726       +1     

Impacted Files	Coverage Δ
rapidash.go	70.97% <60.00%> (-0.21%)	⬇️
first_level_cache.go	78.92% <66.66%> (-0.19%)	⬇️
second_level_cache.go	69.30% <84.61%> (-0.10%)	⬇️
query_builder.go	81.33% <95.34%> (+0.07%)	⬆️
option.go	97.50% <100.00%> (+0.06%)	⬆️

---

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[codecov-commenter]

Codecov Report

Merging #30 into master will decrease coverage by 0.03%.
The diff coverage is 78.63%.

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
- Coverage   71.55%   71.52%   -0.04%     
==========================================
  Files          21       21              
  Lines        8772     8786      +14     
==========================================
+ Hits         6277     6284       +7     
- Misses       1770     1776       +6     
- Partials      725      726       +1     

Impacted Files	Coverage Δ
rapidash.go	70.97% <60.00%> (-0.21%)	⬇️
second_level_cache.go	69.15% <60.00%> (-0.25%)	⬇️
first_level_cache.go	78.92% <66.66%> (-0.19%)	⬇️
query_builder.go	81.19% <92.06%> (-0.07%)	⬇️
option.go	97.50% <100.00%> (+0.06%)	⬆️

---

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[yosuke310]

Will you do not support for benchmarking with postgres?
It looks like it can be handled by doing the following.

・Use driver values
・Branch CREATE TABLE syntax
・Branch placeholders syntax

[yacchiDAO]

Hi, initDB() in tx_test.go seems to be not supported posgresql.
(I can't execute these tests.)
So I want you to be able to select the driver here.