chore: ensure onStart is called with a writeable span

[dyladan]

Fixes #1620

Also adds tests to guarantee onStart and onEnd are called synchronously when spans are started and ended as required by specification.

[dyladan]

@arminru

[codecov]

Codecov Report

Merging #1621 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1621   +/-   ##
=======================================
  Coverage   91.19%   91.19%           
=======================================
  Files         164      164           
  Lines        5033     5033           
  Branches     1026     1026           
=======================================
  Hits         4590     4590           
  Misses        443      443           

Impacted Files	Coverage Δ
...es/opentelemetry-tracing/src/MultiSpanProcessor.ts	100.00% <100.00%> (ø)
...ges/opentelemetry-tracing/src/NoopSpanProcessor.ts	42.85% <100.00%> (ø)
...telemetry-tracing/src/export/BatchSpanProcessor.ts	91.93% <100.00%> (ø)
...elemetry-tracing/src/export/SimpleSpanProcessor.ts	83.33% <100.00%> (ø)