Limit amount of memory allocated for incremental buffer

Limit amount of memory allocated for incremental buffer

Description
~~~~~~~~~~~

  The amount of memory allocated for incremental buffer, both for backup
  and restore depending on page size for given tablespace and with large
  pages may become as large as 1G.

  It may be even worse when backing up with `--parallel=' option as each
  thread will have its own buffer.

  We need to do something about it. It may even require format changes
  for =.delta= files.