* topology: mark a volume crowded only if it can take writes
Crowding asks for more room to write into, and the writable-volume refresh loop
marked anything past the threshold regardless of whether writes could land
there. Growth already discounts those by intersecting the crowded set with the
writable list, so the entries changed no decision and only took space -- in a
tiered cluster, one for nearly every volume.
* topology: wait for the crowded-volume collector before reading what it saw
Closing the stop channel does not order the collector's writes against the
test's reads.
* topology: drop the sleep from the crowded-volume test
The channels are unbuffered, so every send has been received by the time the
sweep returns, and waiting for the collector covers the recording. The sleep
only suggested the result turned on timing.
