Port MongoChem RPC to new MoleQueue JsonRpc

This ports the MongoChem RPC code to use the new JsonRpc class
in the MoleQueue library. Also removes the usage of JsonCpp in
favor of Qt5Json.

Change-Id: Ie2d60c1781f296ba8aa7f3b2782db88498dd3f1a
Forward declare JsonRpc, remove unnecessary includes

Change-Id: Ibcb3b291c5f2834e7ef89dd7bd31e1d2c1e2a01a