Fix some missing durable writes in CheckpointBuilder (#5080)

This hopefully fixes a crash we're seeing in the field, with a signature
like: `std::runtime_error("Corrupt checkpoint file
/var/lib/stellar/buckets/history/ledger/ledger-0398c97f.xdr.dirty, ends
on ledger 60344665, LCL is 60344667")`

The fix is to change a couple `writeOne` calls to `durableWriteOne`
calls in the `CheckpointBuilder`. As far as I can tell their absence was
just an oversight during the multiple iterations of development of the
original `CheckpointBuilder` PR #4446 -- initially the PR didn't have
`durableWriteOne`, and then later after some discussion about durability
guarantees it gained that path, but only 2 of the 4 cases of `writeOne`
in the code got updated to use it.

By not doing durable writes here, it's possible for core to lose a
suffix of a dirty checkpoint file, which in turn can violate the safety
invariant that the dirty checkpoint files are always _ahead_ of the
sqlite-committed LCL. The `CheckpointBuilder` detects this invariant
violation on startup and crashes with the message above.

Why this started to manifest only in v25 is a _bit_ unclear, but there
are a few possibilities. As a statistical sort of thing, the machine
that failed in the field might just have got unlucky: its OS failed to
write a buffer either on a crash or other non-graceful shutdown
condition. Alternatively it _might_ be due to the fact that we removed
some other unrelated fsync calls in general recently, for example in
#4952, which might have coalesced with the flushed-but-not-fsync'ed
`CheckpointBuilder` files. It's hard to be sure about anything with
fsync! But I think this change ought to at least improve the odds of
`CheckpointBuilder` maintaining its invariants.

(The removed `flush` call after the previous `writeOne` is intentional
-- flushing the stream buffer happens as part of `durableWriteOne`)

Author: sisuresh

src/history/CheckpointBuilder.cpp

@@ -170,8 +170,7 @@ CheckpointBuilder::appendLedgerHeader(LedgerHeader const& header,
     LedgerHeaderHistoryEntry lhe;
     lhe.header = header;
     lhe.hash = xdrSha256(header);
-    mLedgerHeaders->writeOne(lhe);
-    mLedgerHeaders->flush();
+    mLedgerHeaders->durableWriteOne(lhe);
 }
 
 namespace
@@ -289,7 +288,7 @@ CheckpointBuilder::cleanup(uint32_t lcl)
                                   ft.localPath_nogz_dirty(), lcl);
                         break;
                     }
-                    out.writeOne(entry);
+                    out.durableWriteOne(entry);
                 }
                 catch (xdr::xdr_runtime_error const& e)
                 {