Fix Pool bug introduced in 2add1c68ead52e67d2f821392302a49bc192b537
Fix Pool bug introduced in 2add1c68ead52e67d2f821392302a49bc192b537

I think this is the bug behind this failed test suite: https://travis-ci.org/groue/GRDB.swift/jobs/253093331

In 2add1c68ead52e67d2f821392302a49bc192b537 Pool has started relying on *indexes* inside its inner array of items. Those indexes weren't reliable enough: this commit brings back independence from indexes by making the Item type a class again.

groue/GRDB.swift,groue/GRDB.swift,groue/GRDB.swift,groue/GRDB.swift