Consistent index start

[liamsi]

Provide a function (or several) in a central place that takes care of indices/revisions/numbers/epochs such that epochs can start at 0 without error'ing and without making the dev think about it in every single call.

[gdbelvin]

Related to #756

[gdbelvin]

Proposed solutions:

• The trillian.LogClient that KT uses should wrap the necessary calls and provide a MapRevision based API that does the appropriate translation.
• We could insert a proper 0 entry into the Log to get us started and make the map revision equal to the log index.
  • No translation layer would be needed
  • Since the 0 entry is an empty struct, we could keep the current API which interprets requests for revision 0 as requests for the current epoch.
  • Index 0 would be the object hash of an empty SignedMapRoot struct - a well known value.
  • Index 1 would be the hash of the first signed map root of an empty map.
  • Index 2 would be the first to location to contain user data.