chatmail-metadata warns about unknown requests

[link2xt]

There are many warnings that look like this:

Sep 03 02:20:20 chatmail-metadata[2795551]: WARNING:root:unknown dictproxy request: 'U1\tpriv/909972312f56d666527204003dd4fc98/comment'
Sep 03 02:20:20 chatmail-metadata[2795551]: WARNING:root:unknown dictproxy request: 'U2\tshared/909972312f56d666527204003dd4fc98/comment'
Sep 03 02:20:21 chatmail-metadata[2795551]: WARNING:root:unknown dictproxy request: 'U3\tpriv/28b3ff10bd55d6663c5604003dd4fc98/comment'
Sep 03 02:20:21 chatmail-metadata[2795551]: WARNING:root:unknown dictproxy request: 'U4\tshared/28b3ff10bd55d6663c5604003dd4fc98/comment'
Sep 03 02:20:34 chatmail-metadata[2795551]: WARNING:root:unknown dictproxy request: 'U1\tpriv/e8989a394356d666c47504003dd4fc98/comment'
Sep 03 02:20:34 chatmail-metadata[2795551]: WARNING:root:unknown dictproxy request: 'U2\tshared/e8989a394356d666c47504003dd4fc98/comment'
Sep 03 02:20:47 chatmail-metadata[2795551]: WARNING:root:unknown dictproxy request: 'U1\tpriv/707f35175356d666d77704003dd4fc98/comment'
Sep 03 02:20:47 chatmail-metadata[2795551]: WARNING:root:unknown dictproxy request: 'U2\tshared/707f35175356d666d77704003dd4fc98/comment'
Sep 03 02:20:48 chatmail-metadata[2795551]: WARNING:root:unknown dictproxy request: 'U3\tpriv/2808e30f0756d666016a04003dd4fc98/comment'
Sep 03 02:20:48 chatmail-metadata[2795551]: WARNING:root:unknown dictproxy request: 'U4\tshared/2808e30f0756d666016a04003dd4fc98/comment'

U means "unset" according to https://doc.dovecot.org/2.3/developer_manual/design/dict_protocol/#dovecot-dict-protocol
As far as I see we don't handle "unset" at all.

Not sure what causes these unset commands. Maybe this happens when user or folder is created. I think we should handle "unset" for all keys by doing nothing and reporting "O" (ok) status or "F" if it is some key that we want to return and make it impossible to unset.