norev message sequence has wrong property name

The norev message expects sequence to be included as a "sequence", but we're actually writing it with the "seq" property name.

This has implications for ISGR checkpointing that use this field, so the following changes will have to be made:

• If protocol version 2 and client type ISGR - read and write value as "seq"
• Otherwise use "sequence"

If this is being backported to 2.x, we can't do the protocol version check, and should just do a client type check.

https://github.com/couchbase/couchbase-lite-core/blob/71cb99144e0af656608f49680a0ff46f48f28c33/modules/docs/pages/replication-protocol.adoc#norev