Make idle events per-partition, but leave Instance::EmitIdle() and its
underlying idle_monitor which broadcasts idle events to all
partitions.

For efficient traversal within one partition, e.g. for distributing
partition-local idle events.

Caused by a revert accident in commit
f2cdbeac

Closes https://github.com/MusicPlayerDaemon/MPD/issues/631

This reverts commit 58d7804d.  It
caused a use-after-free bug when Client::OnSocketError() was called
due to a failed write, e.g. if the output buffer was full.

Clarify that those can't throw, preparing for the next commit.

Use the same chunk size for all binary commands.

Move code from read_stream_art().

Allows removing a CommandResult::CLOSE check.

This catches lines within command lists as well.