This PR fixes an issue in traceChain, where the statedb Commit operation was performed asynchronously with dereference-operations agains the underlying trie.Database instance. Due to how the reference counting works within the trie database (where parent count is recursively updated when new parents are added), doing dereferencing in the middle of Commit can cause the refcount to become wrong, leading to an inconsistent state.
This was fixed by doing Commit/Deref from the same routine.
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.
