wtdb+wtclient: segregate session-key-index reservations by blob-type

This commit introduces a change in the key format used to reserve/lookup
session-key-indexes. Currently the reservations are stored under the
tower id, however this creates issues when multiple clients are using
the same database since only one reservation is permitted per tower.

We fix this by appending the blob type to the session-key-index locator.
This allows multiple clients to reserve keys for the same tower, but
still limits each client to one outstanding reservation. The changes are
made in a way such that we fall back to the legacy format if the a
reservation under the new format is not found, but only if the blob type
matches blob.TypeAltruistCommit, which is so far the only actively
deployed blob type.

watchtower/wtclient/session_negotiator.go

@@ -298,7 +298,9 @@ retryWithBackoff:
 		// Before proceeding, we will reserve a session key index to use
 		// with this specific tower. If one is already reserved, the
 		// existing index will be returned.
-		keyIndex, err := n.cfg.DB.NextSessionKeyIndex(tower.ID)
+		keyIndex, err := n.cfg.DB.NextSessionKeyIndex(
+			tower.ID, n.cfg.Policy.BlobType,
+		)
 		if err != nil {
 			log.Debugf("Unable to reserve session key index "+
 				"for tower=%x: %v", towerPub, err)