Move wallet scanning code into WalletScanner

[NullSoldier]

Summary

This new wallet scanner tries to move some of the code (excluding decryption) into a new class called the WalletScanner. This will act as the holding ground for new scanning related code. This doesn't exactly clean up a lot of the code, but does start as the beginning of an attempt to simplify the scanning code.

• Most scanning code now exists in WalletScanner
• Merged Wallet.scanTransactions() into Wallet.scan()
• Renamed Wallet.updateHead() to Wallet.scan()
• The ChainProcessor is no longer exposed publically
• The wallet head now no longer returns anything meaningful in the getStatus() RPC.
• Effectively deleted wallet/rescanAccount in favor of wallet/rescan. It rescans all accounts and doesn't apply to one account. Also changed request input.
• wallet/rescan nowalways aborts a previous scan no matter what. This is to make the purpose of this command more simple. If you call rescan it WILL rescan.

Fixes IFL-2570

Testing Plan

1. Start scanning
2. Cause rescan
3. Disable an account
4. Start another rescan

Documentation

[X] Yes

Documentation added here https://github.com/iron-fish/website/pull/704

Breaking Change

[x] Yes

• Replaced wallet/rescanAccount RPC with wallet/rescan
• RPC node/getStatus accounts.head.hashandaccounts.head.sequenceare always""and-1` respectively. The head and sequence is no longer returned because there is no longer a single wallet head. Each account has their own head.

[dguenther]

This needs doc changes at least queued up before it goes in I think