Make storage path configurable

[kxcd]

When starting the dash-spv-ffi I see the following warning about the storage path.

WARN dash-spv FFI config missing storage path, falling back to temp dir "/tmp/dash-spv/dash"

Looking at the options, I don't see any way to set a new location.

Also, the default location should be moved from /tmp/ for the following reasons.

• It will be wiped after a reboot
• Another user on the system that tries to run the app will find they are unable to write to that directory causing an error.
• The /tmp partition is very small and may run out of space.

Suggestion: Save the data in the user's home directory, eg ~/.dash-spv.

Also, I notice the app refers to mainnet internally as dash, this is non-standard, we should call mainnet by it's name, so the sub-dir in the data folder should also be named mainnet eg ~/.dash-spv/mainnet.