Significant load-time improvements for startup & order book

[naezith]

This branch has optimizations for the portfolio and orderbooks, with the goal of the near-instant opening of those pages.

Changes:

• Optimized the loop inside the loadWalletSnapshot
• Db.getCurrentWallet() is now passed into loadWalletSnapshot, instead of querying it inside
• loadWalletSnapshot is called a lot earlier at launch, for faster portfolio initialization
• Optimized getWalletSnapshot DB query
• Add Coin buttons on the Portfolio page now hides the loading spinner until the user taps on them
• All Orderbooks of the enabled coins with portfolio balance are prefetched for instant access later on
• Bulk-fetching Orderbook requests complete a lot faster, as they running asynchronously now
• Orderbooks are prefetched at launch, and every minute
• OrderbookDepths are not fetched anymore. Instead, they are calculated from the Orderbooks
• Added functionality to save and load Orderbook Snapshots. Currently not in use since they are being fetched anyways

Needs deep testing.

[naezith]

@kivqa Could you please test these changes? Very critical stuff.