Enforce reasonable timeouts for all routines

[foxcpp]

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.