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/lru_string_map.hh

@@ -136,7 +136,7 @@ public:
                 _map.erase(_map.begin());
             }
             if (++ret % evictions_per_yield == 0) {
-                co_await coroutine::maybe_yield();
+                co_await seastar::coroutine::maybe_yield();
             }
         }
         co_return ret;