storage_proxy: place unique_response_handler:s in small_vector instea…

…d of std::vector

This cuts an allocation in the write path. Instruction count reduction isn't
large, but performance does improve (results are consistent):

before: 196369.48 tps ( 55.2 allocs/op,  13.2 tasks/op,   51658 insns/op)
after:  199290.32 tps ( 54.2 allocs/op,  13.2 tasks/op,   51600 insns/op)

(this is perf_simple_query --write --smp 1 --operations-per-shard 1000000)

service/storage_proxy.hh

@@ -66,6 +66,7 @@
 #include "cdc/stats.hh"
 #include "locator/token_metadata.hh"
 #include "db/hints/host_filter.hh"
+#include "utils/small_vector.hh"
 
 class reconcilable_result;
 class frozen_mutation_and_schema;
@@ -199,7 +200,7 @@ private:
         ~unique_response_handler();
         response_id_type release();
     };
-    using unique_response_handler_vector = std::vector<unique_response_handler>;
+    using unique_response_handler_vector = utils::small_vector<unique_response_handler, 1>;
     using response_handlers_map = std::unordered_map<response_id_type, ::shared_ptr<abstract_write_response_handler>>;
 
 public: