[Storage] Fix childspan context while tracing cassandra queries

[annanay25]

Which problem is this PR solving?

• PR fixes childSpan context in the cassandra query tracer.

Short description of the changes

• Child-spans should be created with context, which was missing earlier.

[codecov]

Codecov Report

Merging #1131 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1131   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files         142     142           
  Lines        6743    6743           
======================================
  Hits         6743    6743

Impacted Files	Coverage Δ
plugin/storage/cassandra/spanstore/reader.go	100% <100%> (ø)	⬆️

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[black-adder]

changes look good, you'll have to sign all commits though: https://github.com/jaegertracing/jaeger/blob/master/CONTRIBUTING.md#sign-your-work

[yurishkuro]

thanks!

[yurishkuro]

please git commit --amend -s to add the sign-off

[annanay25]

Thanks!