Added InfluxDB.flush() and consistency level can be configured for batching

[jganoff]

This addresses a couple requested enhancements described in #289. Specifically:

1. 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. This doesn't change the failure handling for BatchProcessor.write() so records may still be silently dropped as a result of flushing.
2. The consistently level can be configured for asynchronous writes. The default was not changed and is still ONE. It can be explicitly set via InfluxDB.enableBatch(int, int, TimeUnit, ConsistencyLevel).

[codecov-io]

Codecov Report

Merging #290 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #290      +/-   ##
============================================
+ Coverage     78.47%   78.47%   +<.01%     
- Complexity      130      133       +3     
============================================
  Files            11       11              
  Lines           706      720      +14     
  Branches         77       77              
============================================
+ Hits            554      565      +11     
- 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	98.05% <100%> (+0.14%)	17 <1> (+1)	✅
src/main/java/org/influxdb/impl/InfluxDBImpl.java	79.8% <100%> (-0.79%)	41 <4> (+2)

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[majst01]

Hi @jganoff

this is a good start, but i want to seperate these 2 modification into 2 seperate PR's to make them easier to review and accept.

[jganoff]

Sure, I'll split them and send an update.