Trace signals conditionally exposed, take 2

[maackle]

PR summary

Redo of #1378

• Rename Signal::Internal to Signal::Trace
• Add expose_trace_signals bool to Configuration, if false then Trace signals will never be emitted over any interface
• Set expose_trace_signals to true for nodejs conductor config

The first commit is noisy, it's just the sweeping rename to Trace. Look at the second commit for the actual filtering logic

changelog

Please check one of the following, relating to the CHANGELOG-UNRELEASED.md

• this is a code change that effects some consumer (e.g. zome developers) of holochain core so it is added to the CHANGELOG-UNRELEASED.md (linked above), with the format - summary of change [PR#1234](https://github.com/holochain/holochain-rust/pull/1234)
• this is not a code change, or doesn't effect anyone outside holochain core development