go: Start of ZODB

Our path of implementing NEO in Go will be not only for server-side, but
also for client-side, since it is needed by Wendelin.core. On
server-side we'll also need to work with types and data model Python
ZODB implementation uses, so here it goes: Start of ZODB in Go.

Here we define ZODB data types, data model and operational interfaces
for IStorage + friends.

The interfaces are currently read-only with stubs for write mode.