Various improvements coming out of stress testing

[shamsasari]

Created a simple stress testing tool which can submit a large number of vote requests across all active polls on the network. It takes a start and end derivation index, and uses a base mnemonic secret phrase to create a large number of accounts. The only manual process is assigning tokens with the faucet. The tool invokes the client via its command line interface. This required moving all of the client logic from its main.rs into lib.rs. The verification logic was moved into its own file.

The following improvements were made as a result:

• Dealing with rate limiting from subscan by retrying after Retry-After seconds (if the header is present)
• Returning 429 Too Many Requests status code on /vote and /remove-vote if the DynamoDB operation errors with ProvisionedThroughputExceededException or RequestLimitExceeded. The client has been updated with retry logic to deal with this.
• Using subscan to retrieve extrinsic sender account when determining the voters for a poll (which is done to check for clients voting outside of Glove), instead of using the system BlockHash storage. Node endpoint services (or at least some of them) seem to be discarding blocks after a certain amount of time and can't be relied upon for this.
• This extrinsic sender account information is cached globally to prevent it being retrieved repeatedly by the background thread (which runs every 60s)

The node_endpoint field from /info has also been removed. Some endpoint services require an API key, which would be part of the URL, and thus having the endpoint in /info would be leaking it. Clients can pick their own endpoint using the network_name field.