safekeeper: use max end lsn as start of next batch (#11152)

## Problem

Partial reads are still problematic. They are stored in the buffer of
the wal decoder and result in gaps being reported too eagerly on the
pageserver side.

## Summary of changes

Previously, we always used the start LSN of the chunk of WAL that was
just read. This patch switches to using the end LSN of the last record
that was decoded in the previous iteration.

libs/postgres_ffi/src/lib.rs

@@ -396,6 +396,14 @@ pub mod waldecoder {
             self.lsn + self.inputbuf.remaining() as u64
         }
 
+        /// Returns the LSN up to which the WAL decoder has processed.
+        ///
+        /// If [`Self::poll_decode`] returned a record, then this will return
+        /// the end LSN of said record.
+        pub fn lsn(&self) -> Lsn {
+            self.lsn
+        }
+
         pub fn feed_bytes(&mut self, buf: &[u8]) {
             self.inputbuf.extend_from_slice(buf);
         }