Implement journal write batching and slightly refactor journal writes

Slightly reduces WA. For example, in 4K T1Q128 replicated randwrite tests
WA is reduced from ~3.6 to ~3.1, in T1Q64 from ~3.8 to ~3.4.

Only effective without no_same_sector_overwrites.

src/blockstore_impl.cpp

@@ -235,6 +235,12 @@ void blockstore_impl_t::loop()
         {
             throw std::runtime_error(std::string("io_uring_submit: ") + strerror(-ret));
         }
+        for (auto s: journal.submitting_sectors)
+        {
+            // Mark journal sector writes as submitted
+            journal.sector_info[s].submit_id = 0;
+        }
+        journal.submitting_sectors.clear();
         if ((initial_ring_space - ringloop->space_left()) > 0)
         {
             live = true;