Instead of the Sleep function pointer being a standalong global
variable, move it into a new Time struct, and add a Now function
pointer, so that time.Now() can be mocked. Time.Now() isn't used yet.
This will be cleaner if we need to mock more time-related functions in
the future.
PR 320 introduced a bug that causes the `blocks` and `lengths` database
files to be located one directory level higher than it they should be.
The bug doesn't cause any functional problem, it only makes the
lightwalletd do more work (re-download the block cache), and it also
makes it not possible to switch between testnet and mainnet.
This patch locates the database files back where they belong.
Larry Ruane