utils, db: qualify seastar::coroutine:: to avoid shadowing by utils::coroutine class

Inside namespace utils, unqualified coroutine:: resolves to the
utils::coroutine class (utils/coroutine.hh) rather than the
seastar::coroutine namespace. This causes build failures when
replica/database.hh is added to the precompiled header, because
utils/coroutine.hh becomes transitively visible in all TUs.

Qualify all coroutine:: references with seastar:: in affected files
under utils/ and db/.

utils/logalloc.cc

@@ -438,7 +438,7 @@ private:
                 break;
             }
             _reclaim(free_memory_threshold - memory::free_memory());
-            co_await coroutine::maybe_yield();
+            co_await seastar::coroutine::maybe_yield();
         }
         llogger.debug("background_reclaimer::main_loop: exit");
     }