This has not been extensively tested yet, because some other part of the
code breaks in "ibtestblob".

btr_free_page_low(): Add parameters "space" and "page_no", because they
are omitted from compressed BLOB pages.

btr0cur.c: Implement the compression and decompression of BLOB columns,
enabled at compile-time (#define ZIP_BLOB TRUE) for now.

btr_rec_free_externally_stored_fields(),
btr_copy_externally_stored_field(): Made static

mlog_log_string(): New function, split from mlog_write_string(), allows
to avoid a dummy memcpy() of compressed BLOB pages.