example: make db connectors an optional dependency

Move database connectors to `optionalDependencies` field of the
scaffolded `package.json`. This way the scaffolded project can be run
against the in-memory db on the machines where build chain (node-gyp)
is not configured correctly.

/to @raymondfeng please review
/cc @crandmck FYI, we have agreed on this change yesterday.