MEM_HEAP_BTR_SEARCH: Remove. Let us handle this special type of
mem_heap_t allocations in the only compilation unit, btr0sea.cc.

mem_block_info_t::ahi_block: Replaces free_block. This caches one
buffer page for use in adaptive hash index allocations. This is
protected by btr_search_sys_t::partition::latch. It only is
Atomic_relaxed because btr_search_free_space() is following a
pattern of test, lock, and test.

btr_search_check_free_space(): Protect the ahi_block with a
shared AHI partition latch. We must recheck btr_search_enabled after
acquiring the latch in order to avoid a race condition with
btr_search_disable(). Using a shared latch instead of an exclusive one
should reduce contention with btr_search_guess_on_hash() and other
operations when running with innodb_adaptive_hash_index=ON.

This has been tested by running the regression test suite
with the adaptive hash index enabled:
./mtr --mysqld=--loose-innodb-adaptive-hash-index=ON