Users should be able to configure the default location for @truffle/db persistence

[gnidan]

Issue

Right now, @truffle/db by default stores everything in the global Truffle config directory (~/.config/truffle or such on Linux, other directories on other platforms). This can cause trouble because @truffle/db remains a bit experimental, and so users might not want to risk collision or breakages and may instead prefer to store their persistent records on a per-project basis.

This issue proposes that we add a user-level config flag to favor per-project persistence over the current default of global persistence.

Maybe this config looks like:

{
  /* ... */
  db: {
    defaultPersistPerProject: true // this is a terrible name
  }
}