Add option to set an upper bound for memory allocations

[passaro]

Tell us more about this new feature.

Mountpoint should allow users to configure an upper limit for the memory it allocates when serving read and write requests.
The limit would cover all memory buffers that Mountpoint uses when:

• downloading from or uploading to the bucket,
• loading from or writing to the local disk cache,
• downloading from or uploading to the shared cache.

It would not cover memory used to cache metadata.