msg/async/ProtocolV2: pass `desc` as `std::string_view` to write()

msg/async/ProtocolV2: pass `desc` as `std::string_view` to write()

All callers really pass a C string literal, and declaring a
`std::string` parameter will implicitly create two `std::string`
instances: one on the caller's stack, and another one inside write()
as parameter to the continuation lambda.  This causes considerable and
unnecessary overhead.

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>