Remove access to txpool by relying on eth_getTransactionCount for pending txs

[mikiquantum]

go-ethereum has fixed the issue here: ethereum/go-ethereum#2880

We can go ahead and test that fix out, and if successful replace the txpool access logic from the code. In addition, the txpool config flag will not be needed.

This will allow us to integrate with our ethereum providers that implement standard rpc calls.
Parent topic: #403

Testing

• The change needs to be tested on all 3 centrifuge testnets russianhill, bernelheights, dogpatch using Testworld.

Acceptance Criteria

Tests pass on all testnets

[mikiquantum]

We can remove the access to the txpool completely so it is compatible with infura api set:

• This will work on a self hosted node (single node access)
• Need to verify how that would work for infura infra type. The reason for it is that behind the infura api, a read request can go to a different node than the one that the write request can go. In which case, I am not sure how infura can make the txpool (internal) available for all nodes within the cluster, so it is is possible that a retrial mechanism needs to remain (as is right now)

I believe that we can tackle this and remove access to txpool directly and remove config flag and leverage the fixed api transactionCount(pending) for it.