Update rekor REST API to match Trillian semantics (#250)

This patch removes the /api/v1/log/entries/{uuid}/proof endpoint. If you
have the UUID (aka the leaf Merkle hash), you likely want proof that the
content represented by that hash is included in the log. There's no need
for a separate /proof endpoint to deliver the same content.

This commit also ensures that the getLogEntryByIndex and
getLogEntryByUUID endpoints return an inclusion proof as part of their
response content. The search endpoint also now returns the inclusion
proof of all entries returned from the query.

With this patch, Rekor no longer uses the deprecated `GetLeavesByHash`
Trillian API.

Fixes #229

Signed-off-by: Bob Callaway <bob.callaway@gmail.com>