[PR-582] Move buffer deletion to a background task

[justjanne]

In Short

• Adds a background task handler object to each core session
• Moves buffer deletion to that handler

Impact

Criteria	Rank	Reason
Impact	★☆☆ 1/3	Prevents the core from timing out while deleting a large buffer
Risk	★★☆ 2/3	Only changes buffer deletion, which is not critical functionality
Intrusiveness	★☆☆ 1/3	Changes localized to a few classes

Rationale

Currently, while deleting buffers, the core may hang and the IRC connection can time out. With the end of freenode this has become a more pressing matter recently. This PR moves deletion of buffers to a thread per-user to prevent users causing a DoS while simultaneously preventing the core from hanging.