Support for UnsignedLong in GoRPC lib

Java BSON library does not support unsigned longs, but we use uint64 in
many places - keyspaceids, sequence id, etc. Currently uint64s are
decoded as signed longs. With this change we make 0x63 a
valid type and add support for encoding and decoding
UnsignedLong. Also, update references to Seq to use UnsignedLong instead
of long.