Add query-only mode flag

[mmsqe]

Closes: #1359

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[yihuang]

In the context of ethermint, probabely we need to have a mode that start grpc and json-rpc together, but could leave that to another issue.

[mmsqe]

In the context of ethermint, probabely we need to have a mode that start grpc and json-rpc together, but could leave that to another issue.

maybe just query-only flag, we can pass like grpc|jsonrpc|grpc,jsonrpc

[yihuang]

In the context of ethermint, probabely we need to have a mode that start grpc and json-rpc together, but could leave that to another issue.

maybe just query-only flag, we can pass like grpc|jsonrpc|grpc,jsonrpc

or --json-rpc-only which implies all the dependency services.

[adisaran64]

think it might be a good idea to use a bool flag instead of a string for the query-only mode, seems to simplify the code and use?

[fedekunze]

I'm not sure the JSON-RPC only flag will work properly without tendermint. The --grpc-only looks good to me