[swarming] Print all goroutine stacktraces for collect
[swarming] Print all goroutine stacktraces for collect

This will make it easier to debug what seem to be deadlocks in collect
calls that use the new `-eager` flag.

Change-Id: I45633c786306ff73ae23d87400741c5f836952c0
Bug: fuchsia:50776
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2167222
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Oliver Newman <c98d05fdc4005e0f75e160701d6b971b76237a97@google.com>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go