larger item metadata stream chunks, fixes #1452

increasing the mask (target chunk size) from 14 (16kiB) to 17 (128kiB).
this should reduce the amount of item metadata chunks an archive has to reference to 1/8.
this does not completely fix #1452, but at least enables a 8x larger item metadata stream.

borg/archive.py

@@ -39,7 +39,7 @@ HASH_MASK_BITS = 21  # results in ~2MiB chunks statistically
 CHUNKER_PARAMS = (CHUNK_MIN_EXP, CHUNK_MAX_EXP, HASH_MASK_BITS, HASH_WINDOW_SIZE)
 
 # chunker params for the items metadata stream, finer granularity
-ITEMS_CHUNKER_PARAMS = (12, 16, 14, HASH_WINDOW_SIZE)
+ITEMS_CHUNKER_PARAMS = (15, 19, 17, HASH_WINDOW_SIZE)
 
 has_lchmod = hasattr(os, 'lchmod')
 has_lchflags = hasattr(os, 'lchflags')