Improve opencensus-contrib-zpages package

[bogdandrutu]

• Better test coverage for all Z-Pages (tracez, traceconfigz).
• Add support for changing trace params for a period of time (example for the next 5 minutes). Should allow only one temporary change at a time and change the page to display if there is a temporary change in progress, allow to revert that change.
• Tracez page should allow to pick the error code when display the errors.
• Cleanup code in Tracez page by resolving TODOs.
• Cleanup code in TraceConfigz page by resolving TODOs.

[bogdandrutu]

• Add possibility to registe/unregister span names in SampledSpanStore to Traceconfigz.