Add ForceFlush() method to SpanProcessor interface

[Aneurysm9]

• Add a stub implementation to SimpleSpanProcessor
• Add a working implementation to BatchSpanProcessor

Fixes #1072

[codecov]

Codecov Report

Merging #1166 into master will decrease coverage by 0.8%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #1166     +/-   ##
========================================
- Coverage    77.9%   77.0%   -0.9%     
========================================
  Files         135     135             
  Lines        7192    5867   -1325     
========================================
- Hits         5603    4521   -1082     
+ Misses       1346    1104    -242     
+ Partials      243     242      -1     

Impacted Files	Coverage Δ
sdk/trace/batch_span_processor.go	78.4% <ø> (-0.1%)	⬇️
sdk/trace/simple_span_processor.go	66.6% <ø> (-13.4%)	⬇️
sdk/trace/trace_go11.go	50.0% <0.0%> (-10.0%)	⬇️
sdk/resource/resource.go	57.5% <0.0%> (-8.6%)	⬇️
api/trace/api.go	57.1% <0.0%> (-8.4%)	⬇️
api/metric/numberkind_string.go	16.6% <0.0%> (-8.4%)	⬇️
api/metric/must.go	41.3% <0.0%> (-8.2%)	⬇️
internal/testing/alignment.go	37.5% <0.0%> (-8.0%)	⬇️
sdk/metric/aggregator/sum/sum.go	68.0% <0.0%> (-7.8%)	⬇️
... and 127 more

[MrAlias]

Approach looks good. Only question is a small one related to testing.