Add chats_fetch to find a conversation's chatId

[sebpretzer]

Stacked on #3 (helper extraction). Follow-up to mattt#251.

messages_fetch can now be scoped with chatId, but the only way to get one was the isPartOf of messages fetched through the sender-based participants filter, which mixes in unrelated chats. chats_fetch finds the conversation first.

Inputs

• participants: handles of the other people in the chat (you are never in the list)
• match: all (default), exact or any
  • all: the chat includes every participant and may include others
  • exact: the chat includes exactly those people
  • any: the chat includes at least one of them
• query: case-insensitive match on the chat name (unnamed chats never match)
• start / end: chats with messages in the window, parsed the same way as messages_fetch
• limit: default 30

Output: a bare array of the Conversation object isPartOf uses, plus dateModified, most recent first. Each @id can be passed as chatId to messages_fetch.

Notes

• Each participant is resolved to its own set of handles. One handle string can match several (with and without a country code), and Madrid's all counts handles, not people. all and exact therefore use one any clause per participant.
• A participant that resolves to no handle returns [] under all and exact, rather than silently dropping that person. Under any it is skipped.
• exact and query are checked on the fetched chats, so chats are read in pages of 1024 until limit is filled or none are left.
• With start/end, dateModified is the latest message within the window (Madrid computes it over the filtered messages).

Testing: builds clean in Debug. Not yet verified against a real chat.db.