structure is reserved for every index in the data dictionary cache.
(Bug #20877)

We could shrink the structure further to three 32-bit words or two
64-bit words by turning the remaining fields to bit-fields.
Unfortunately, the fields are not protected by any mutex, and thus we
would better keep each field aligned to a machine word.

btr_search_t, buf_block_t: Rename "ulint side" to "ibool left_side".
Remove BTR_SEARCH_LEFT_SIDE and BTR_SEARCH_RIGHT_SIDE, and also rename
some local variables and function parameters from "ulint side" to
"ibool left_side".

btr_search_t: Remove the unused fields last_search, n_direction, direction,
and modify_clock.  Remove the unused constants BTR_SEA_NO_DIRECTION,
BTR_SEA_LEFT, BTR_SEA_RIGHT, and BTR_SEA_SAME_REC.

btr_search_t: Remove magic_n unless #defined UNIV_DEBUG.  Turn an assertion
about the magic number into a debug assertion.