Implement index management spec

Another API conformity ticket.

The following existing method will change:

• create_index will no longer "cache" successfully created indexes client side. create_index continues to always attempt to create an index on the server. If the index already exists this is a no-op on the server.

The following method is deprecated:

• ensure_index - The difference between ensure_index and create_index is that ensure_index consults an index "cache" before sending a create index operation to the server. This causes hard to debug race conditions when dropping and immediately re-creating an index, and provides no real benefits. To avoid these problems we're deprecating the method. Use create_index instead.

The following methods are added:

• create_indexes - like create_index but takes a list of index specs and creates multiple indexes. Only supported by MongoDB 2.6 and newer.
• list_indexes - Returns a cursor over all indexes for the given collection.