Reference counting for mounts

Description: TODO

The main problem is the mount cleanup logic at the end of a client's lifecycle where all mounts for a client are unmounted. This can lead to conflicts if, e.g., Client A and B both want a mount for /data/special. In this case /data/special would get unmounted if either A or B drop their connection which may lead to race conditions on concurrent tasks¹.

This was already implemented/ recognized in the executor MR but has yet to be backported to stable.

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@robinc this likely is the source of the bug with missing heaps for MainLight.thy.

1. The mount would be reconstructed on the next call to, e.g., use_theories but any currently running tasks, like session creation, will get disrupted. ↩