Unify ways to communicate by RPC channel

[akolotov]

At this moment there are two ways in the code to call JSON-RPC methods on the Ethereum node (web3 provider) side.

The first way is to call web3 functionality through the web3 object. Example,

https://github.com/poanetwork/bridge-nodejs/blob/c71f4a41b8616d4f158604874cc5cd9616d5e481/src/tx/web3.js#L1-L7

Another way is to send JSON REST query directly to the node:

https://github.com/poanetwork/bridge-nodejs/blob/c71f4a41b8616d4f158604874cc5cd9616d5e481/src/tx/sendTx.js#L38-L57

As it was discussed here: #28 (comment) it is possible to use the web3 object for all kind of operations. It could simplify some things especially the functionality to support failover RPC URL.