Sync only completed writes

Should be a final remaining fix to EC + non-capacitor (non-immediate-commit) write hangs :).

First it was breaking non-EC ("instantly stable") writes because they sometimes
complete out of order which was leading to the following error:

terminate called after throwing an instance of 'std::runtime_error'
  what():  BUG: Unexpected dirty_entry 1000000000001:29480000 v65540 unstable state during flush: 0x151

But it is easily fixed by scanning previous and next dirty_entries in mark_stable.

src/blockstore_write.cpp

@@ -593,7 +593,7 @@ resume_4:
 #endif
         bool is_big = (dirty_it->second.state & BS_ST_TYPE_MASK) == BS_ST_BIG_WRITE;
         bool imm = is_big ? (immediate_commit == IMMEDIATE_ALL) : (immediate_commit != IMMEDIATE_NONE);
-        bool is_instant = ((dirty_it->second.state & BS_ST_TYPE_MASK) == BS_ST_DELETE || (dirty_it->second.state & BS_ST_INSTANT));
+        bool is_instant = IS_INSTANT(dirty_it->second.state);
         if (imm)
         {
             auto & unstab = unstable_writes[op->oid];