[FAB-2579] Fix inconsistencies in ledger package

https://jira.hyperledger.org/browse/FAB-2579



This changeset:

1. Removes the stuttering that was prevalent in filenames and package
names throughout the package
2. Updates function definitions where appropriate
3. Makes the layout of "ram" and "json" implementation consistent with
the "file" one by moving the relevant functions to `factory.go`
4. Switches all panic calls to their logger-equivalents. (We will be
doing this for all packages inside `orderer`.)
5. Per Binh's request: creates paths via the `filepath.Join` call

Change-Id: I918519cf66b3287864560d1fa2e6734b6953d657
Signed-off-by: Kostas Christidis <kostas@christidis.io>