Fix task handling in wireguard mode

[octol]

• Remove the unused reconnect logic in the bandwidth controller and go back to a single unified TaskManager.
• Dispose the shared mixnet client after signal shutdown but before dropping the task manager

This should fix all those unexpected drop (VPN-2849) errors that happen during disconnect.

IMPORTANT: we need to solidly confirm that the reconnect logic is as unused as I think it is. And test it properly on Android

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This change is 

[pronebird]

This needs to be tested.

Reviewed 5 of 5 files at r1.
Reviewable status: all files reviewed (commit messages unreviewed), all discussions resolved (waiting on @octol)