There is no need to depend on the old context package now that the
minimum Go version is 1.7. The move to "context" eliminates our weird
vendoring setup. Some vendored code still uses golang.org/x/net/context
and it is now vendored in the normal way.
This change triggered new vet checks around context.WithTimeout which
didn't fire with golang.org/x/net/context.
These accessors were introduced by light client changes, but
the only method that is actually used is GetNumberU64. This
commit replaces all uses of .GetNumberU64 with .Number.Uint64.
