Added InfluxDB.flush()

[jganoff]

InfluxDB.flush() was introduced as a mechanism for flushing the underlying BatchProcessor without shutting it down. It's a step toward being able to guarantee all asynchronously queued points are delivered at a given time when operating in batch mode. See #289 for more context. This doesn't change the failure handling for BatchProcessor.write() so records may still be silently dropped as a result of flushing.

[codecov-io]

Codecov Report

Merging #292 into master will decrease coverage by -0.24%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #292      +/-   ##
============================================
- Coverage     78.47%   78.23%   -0.24%     
- Complexity      130      132       +2     
============================================
  Files            11       11              
  Lines           706      712       +6     
  Branches         77       77              
============================================
+ Hits            554      557       +3     
- Misses          109      112       +3     
  Partials         43       43

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/org/influxdb/InfluxDB.java	100% <ø> (ø)	0 <0> (ø)	❌
...rc/main/java/org/influxdb/impl/BatchProcessor.java	97.95% <100%> (+0.04%)	17 <1> (+1)	✅
src/main/java/org/influxdb/impl/InfluxDBImpl.java	79.51% <100%> (-1.09%)	40 <2> (+1)

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