Made the blob test not hardcode maxreflen in some places.
Made blob_t constructor take a block size, and added a guarantee that maxreflen is not too big.  This helps exposed problems in #1237.
Made btree_sindex_block_t have a block_magic.
Merge branch 'next' into sam_1237
Removed no-op blob_t construction in initialize_secondary_indexes.
Merge branch 'next' into sam_1237