Update seastar submodule

* seastar 7ebbb26...124467d (4):
  > peering_sharded_service: prevent over-run the container
  > sharded: fix move constructor for peering_sharded_service services
  > sharded: improve support for cooperating sharded<> services
  > sharded: support for peer services

Includes change to batchlog_manager constructor to adapt it to
seastar::sharded::start() change.

Needed for gossip backport.

db/batchlog_manager.hh

@@ -84,9 +84,6 @@ public:
     // to be per shard and does no dispatching beyond delegating the the
     // shard qp (which is what you feed here).
     batchlog_manager(cql3::query_processor&);
-    batchlog_manager(distributed<cql3::query_processor>& qp)
-        : batchlog_manager(qp.local())
-    {}
 
     future<> start();
     future<> stop();