* master: survive a layout deleted twice
Two volume servers dropping the last replica of volumes that share a layout
both find it empty and both delete it. The loser's lookup misses, and the
single-value type assertion on the result crashed the master before the
caller could look at the found flag.
Claude-Session: https://claude.ai/code/session_01WmX6Rchx298NQksHDXg7sk
* master: remove a layout and read it back in one step
DeleteVolumeLayout looked the layout up and then deleted it, so two deleters
could each release the lookup ownership of the same layout, or one could find
nothing to release at all. Have the map hand back what it removed.
Claude-Session: https://claude.ai/code/session_01WmX6Rchx298NQksHDXg7sk
Chris Lu
5f787a25c3