InnoDB crash recovery used a special type of mem_heap_t that
allocates backing store from the buffer pool. That incurred
a significant overhead, leading to underutilization of memory,
and limiting the maximum contiguous allocated size of a log record.

recv_sys_t::blocks: A linked list of buf_block_t that are allocated
by buf_block_alloc() for redo log records. Replaces recv_sys_t::heap.
We repurpose buf_block_t::unzip_LRU for linking the elements.

recv_sys_t::max_log_blocks: Renamed from recv_n_pool_free_frames.

recv_sys_t::max_blocks(): Accessor for max_log_blocks.

recv_sys_t::alloc(): Allocate memory from the current recv_sys_t::blocks
element, or allocate another block.  In debug builds, various free()
member functions must be invoked, because we repurpose
buf_page_t::buf_fix_count for tracking allocations.

recv_sys_t::free_corrupted_page(): Renamed from recv_recover_corrupt_page()

recv_sys_t::is_memory_exhausted(): Renamed from recv_sys_heap_check()

recv_sys_t::pages and its elements are allocated directly by the
system memory allocator.

recv_parse_log_recs(): Remove the parameter available_memory.

We rename some variables 'store_to_hash' to 'store', because
recv_sys.pages is not actually a hash table.

This is joint work with Thirunarayanan Balathandayuthapani.