Route every production account sync through one SyncDispatcher that caps
simultaneous syncs (NUNCIO_SYNC_CONCURRENCY, default 4), coalesces a
duplicate in-flight sync for the same account, applies a per-account
timeout (NUNCIO_SYNC_TIMEOUT, default 300s), and races in-flight syncs
against the shared shutdown signal. The client Mail.Sync RPC and the
daemon's CoreCommand sync consumer now share a single dispatcher instance
so one slow account can no longer head-of-line-block the others.
Refs #292