Enforce reasonable timeouts for all routines

As part of the goal to improve maddy robustness, context.Context should be used
consistently in all interfaces and reasonable timeouts for operations should be enforced
using these.

- [ ] SMTP check methods
- [ ] SMTP modifier methods
- [ ] IMAP commands (server)
- [ ] SMTP commands (server)
- [ ] Delivery target operations

With that considered, maddy should have as much operations as possible implemented
in a rollback-able way so composite operations could be cancelled due to timeout in one of
sub-operations.

Notable considerations:
- [ ] Delivery.AddRcpt should be revertable. At least last call should be.

With current implementation (non-revertable AddRcpt), it is possible that if multiple delivery targets are used
for one recipient, duplicate messages may be delivered in case of partial failures.