Increment nonce count when populating transaction

[emizzle]

Fixes: codex-storage/nim-codex#538

Problem

Concurrently calling sendTransaction (via a contract call, for example) would call getTransactionCount multiple times without the underlying value being incremented before the next call.

Solution

Keep track of a lastSeenNonce in the signer on each call to populateTransaction. On each call, the lastSeenNonce is compared to the getTransactionCount value, and when it is greater, the lastSeenNonce is incremented, otherwise it is set to the getTransactionCount value.

Additionally, if a transaction is crated manually and sent (via sendTransaction), the lastSeenNonce is updated.

[markspanbroek]

Very nice, thanks @emizzle!