source transactions from an Indy Read replica

[rxbryan]

Define a source-indy-read-replica for the Indy read daemon here .
It should implement the source APIs

return {
    getSourceInfo,
    getTxData,
    getHighestSeqno,
    describe
  }

One approach to implementing support for this would be to add an rpc server exposing the methods above to the indy read daemon...