add checkstyle for javadoc, fix javadoc for database v1.2.1

[johnding1996]

Now checkout can guide us to write good javadocs.

[johnding1996]

The build failed of travis is due to the failed of checkstyleMain task since errors exists, looking for methods to allow this.

[codecov[bot]]

Codecov Report

Merging #79 into develop will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##             develop     #79   +/-   ##
=========================================
  Coverage       66.7%   66.7%           
  Complexity       328     328           
=========================================
  Files             36      36           
  Lines           1589    1589           
  Branches         176     176           
=========================================
  Hits            1060    1060           
  Misses           453     453           
  Partials          76      76

Impacted Files	Coverage Δ	Complexity Δ
...-chatbot/src/main/java/database/keeper/Keeper.java	100% <ø> (ø)	3 <0> (ø)	:arrow_down:
...bot/src/main/java/database/connection/SQLPool.java	58% <ø> (ø)	4 <0> (ø)	:arrow_down:
...g-chatbot/src/main/java/utility/TextProcessor.java	80% <ø> (ø)	2 <0> (ø)	:arrow_down:
...src/main/java/database/keeper/SerializeKeeper.java	78.2% <ø> (ø)	10 <0> (ø)	:arrow_down:
.../main/java/database/connection/SQLPoolFactory.java	0% <ø> (ø)	0 <0> (ø)	:arrow_down:
...ain/java/database/connection/RedisPoolFactory.java	0% <ø> (ø)	0 <0> (ø)	:arrow_down:
...t/src/main/java/database/connection/RedisPool.java	52.3% <ø> (ø)	4 <0> (ø)	:arrow_down:

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