[Feature Request] Allow setting the location of the mainnet db files

[SlugFiller]

After less than half a year online, the mainnet SQLite db has already grown to nearly 4gb in size. At the current rate, it will be large enough to rival a plot file in just a couple of years. At the same time, it's currently forced to be stored in ~/.chia/mainnet/db. The home folder is often in the same drive as the OS, which is often a fast yet small SSD, as that guaranteed the fastest startup time (and this is still the good version, it could also be on a ramdisk). However, it can often be far too small to hold the quickly growing db file.

Symlinking can be a workaround on operating systems that support it, but not all do.

It would be much preferable if there was a way to move the database files to a folder of the user's choosing, on a drive of the user's choosing, where there would be sufficient space to hold the growing data needed by a full node.