commitlog: switch to fragmented buffers

So far commitlog was using contiguous buffers for storing the data that
is about to be written to disk. It was able to coalesce small writes so
that multiple small mutations would use the same buffer, but if a
muation was large the commitlog would attempt to allocate a single,
appropriately large buffer. This excessively stresses the memory
allocator and may cause memory fragmentation to become an issue. The
solution is to use fixed-size buffers of 128 kB, which is the standard
buffer size in Scylla and keep large values fragmented.

db/commitlog/commitlog_entry.hh

@@ -71,7 +71,7 @@ public:
         return _mutation.representation().size();
     }
 
-    void write(typename seastar::memory_output_stream<std::vector<temporary_buffer<char>>::const_iterator>::simple& out) const;
+    void write(typename seastar::memory_output_stream<std::vector<temporary_buffer<char>>::iterator>& out) const;
 };
 
 class commitlog_entry_reader {