Only add indexed BLOBs to row_ext.

trx_undo_rec_get_partial_row(): Move the BLOB fetching to row_ext_create().

row_build(): Pass only those BLOBs to row_ext_create() that are referenced by
ordering columns of some indexes, similar to trx_undo_rec_get_partial_row().

row_ext_create(): Add the parameter "tuple".  Move the implementation
from row0ext.ic to row0ext.c.

row_ext_lookup_ith(), row_ext_lookup(): Return a const pointer.  Remove
the parameters "field" and "f_len".  Make the row_ext_t* parameter const.

row_ext_t: Remove the field zip_size.

field_ref_zero[]: Declare in btr0types.h instead of btr0cur.h.

row_ext_lookup_low(): Rename to row_ext_cache_fill() and change the
signature.