add `eden trace hg` command
add `eden trace hg` command

Summary:
Add a TraceBus to HgQueuedBackingStore and allow tracing import events over Thrift.

This powers a new `eden trace hg` command that allows a live view of
tree and blob imports.

Reviewed By: genevievehelsel

Differential Revision: D24732059

fbshipit-source-id: 525152fe39047160a68c1706217a06a00a6dbef1

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