Automatic powcache download

[CodeIsTheKey]

Automatic Powcache Download

For both Smart Nodes and Clients, it would be nice if they would check for an updated powcache file and update the file if it is significantly ahead of any existing powcache data. This would simplify a quick startup for new users.

Notes

• Store and regularly update powcache.dat files for all desired networks. Suggested naming powcache-{network}.dat. We can keep these in the same place we normally store bootstraps.
• On program startup, check the local and remote file sizes. The powcache.dat file is roughly 128bytes/block.
• We can easily estimate how much of a benefit the network file would be. If the benefit is sufficient, download, update, and load the new powcache file.

• Requires Add libcurl to Depends #151
• Add check/update
• Add command line/config option to disable check

[HumbleDeer]

Would a small powershell or Python script be a better temporary idea? It can check and be interactive.