https://jira.hyperledger.org/browse/FAB-2603



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.

Change-Id: I3bdf9763e227c29983003bab0769282136a42511
Signed-off-by: Kostas Christidis <kostas@christidis.io>