feature: #295 Create and Drop retention policies

[ashamanur]

• Created methods to create and drop retention policies.
• Added unit test to test the same.

[codecov-io]

Codecov Report

Merging #351 into master will increase coverage by 0.06%.
The diff coverage is 84.61%.

@@             Coverage Diff              @@
##             master     #351      +/-   ##
============================================
+ Coverage     83.13%   83.19%   +0.06%     
- Complexity      200      207       +7     
============================================
  Files            16       16              
  Lines           907      946      +39     
  Branches         93       96       +3     
============================================
+ Hits            754      787      +33     
- Misses          109      111       +2     
- Partials         44       48       +4

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/org/influxdb/InfluxDB.java	100% <ø> (ø)	0 <0> (ø)	⬇️
src/main/java/org/influxdb/impl/Preconditions.java	63.63% <33.33%> (-11.37%)	7 <1> (+1)
src/main/java/org/influxdb/impl/InfluxDBImpl.java	83.75% <88.88%> (+0.76%)	53 <6> (+6)	⬆️

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[majst01]

Nice, would you mind adding Changelog and Readme additions, then i will merge.

[timhallinflux]

fixes #295