pin sled, write a note explaining why

Cargo.toml

@@ -442,7 +442,12 @@ signal-hook = "0.3"
 signal-hook-tokio = { version = "0.3", features = [ "futures-v0_3" ] }
 sigpipe = "0.1.3"
 similar-asserts = "1.5.0"
-sled = "0.34"
+# Don't change sled's version on accident; sled's on-disk format is not yet
+# stable and requires manual migrations. In the limit this won't matter because
+# the upgrade system will replace the DNS server zones entirely, but while we
+# are still doing mupdate a change to the on-disk format will break existing DNS
+# server zones.
+sled = "=0.34.7"
 sled-agent-client = { path = "clients/sled-agent-client" }
 sled-hardware = { path = "sled-hardware" }
 sled-hardware-types = { path = "sled-hardware/types" }