[SCHEMA] Make Message.*_addr non-null to prevent TypeErrors.

In Thread.participants, we assume that these attributes are iterable, but they
may not be set at all in case of decoding error. It's generally safer to have
them maybe be the empty list, but never null.

Fixes T181.
Fix previous migration down_revision ID.
[SCHEMA] Remove duplicate Gmail messages.
Also create ImapUid entries when deduplicating Message object creation.

Fixes T180, caused by bugs in how we create Gmail imapuids.
Fix bug in g_msgids().

We occasionally were failing to deduplicate message object creation or
download because we wrongly excluded messages which currently had no
associated ImapUids. (This is actually a common situation, as when a
message moves we might well delete the old ImapUid before creating the
new one.)