This moves the tracing RPC API implementation to package eth/tracers.
By doing so, package eth no longer depends on tracing and the duktape JS engine.
The change also enables tracing using the light client. All tracing methods work with the
light client, but it's a lot slower compared to using a full node.
# Conflicts:
# cmd/utils/flags.go
# eth/api.go
# eth/tracers/api.go
# les/api_backend.go