Change and export dir where ledgers are kept

1. Ledgers were maintained as directories inside a "blocks" directory. Semantically speaking, this doesn't quite make sense – you'd expect to find blocks under a "blocks" directory.

2. The name of this directory should be an exported constant so that other packages can access it programmatically. The orderer package for instance, needs this info in order to construct the proper directories for its ledger.

This changeset addresses both of these issues.