When Thunderbird starts, it reads from the calendar databases. A lot. In this bug I propose we delay reading events/tasks from the databases until after Thunderbird has finished starting up, and force operations using the database to wait. The effect on start-up time is marginal, but it does reduce the amount of context-switching that has to happen.

Hopefully in future we can extract the things that want to read the database and not do them at all until after start-up. But this is a good first step.

Depends on D153906

Pushed by geoff@darktrojan.net:
https://hg.mozilla.org/comm-central/rev/3f9bfbd74b0a
Delay most calendar database read operations until after Thunderbird start-up. r=mkmelin