Fix issues related to precaching

[guilhermelawless]

Having received multiple reports of unexpected issues, such as receiving wrong work, a commit 638f22c has been pushed which disables precaching by default. This means all work is calculated on demand.

Benefits of disabling precache:

• No longer needing a connection to a Nano node
• Profoundly reduced complexity
• Avoiding calculating work that won't be used if the account is being used elsewhere

Disadvantages:

• Added latency for incoming requests

Since on demand work has been served with less than a second of latency, I don't see an issue with disabling precaching until the work difficulty increases substantially.

I don't expect to find the time to investigate the root of the problem. I would advise a complete re-write of the server with testing in mind, as this one is not ready for it.