Buffer pool may remove a page from memory, and even free the memory in which it resided unless it is marked as "buffer fixed".
To be more precise: buf_block_t struct itself can become freed if the buffer pool is requested to shrink.

Serveral places in our code dereferenced a buf_block_t pointer either after unfixing it, or before fixing it, which was not safe.

This patch:
- moves the accesses that happened after unfixing, to happen before
- makes sure that places which store buf_block_t pointer as hint for future reference are structured in a way which prevents races with buffer pool resize. That is it introduces Block_hint class which stores a pointer, and allows accessing this pointer only by passing a functor which is run passing either the original pointer  if it was still valid, or a nullptr if the pointer become stale in meantime
- adds more assertions about buf_block_t's page is buffer fixed when accessed

This patch also fixes Bug #31389823	BUF_BLOCK_IS_UNCOMPRESSED() RETURNS FALSE IN TOO MANY CASES, by simply removing this function and using buf_pointer_is_block_field_instance instead.

It also backports important parts of Bug#20735882 REMOVE REDUNDANCY IN mtr0mtr.cc RELATED TO RELEASING MEMO SLOT fix from 8.0

RB: 24661
Reviewed-by: Marcin Babij <marcin.babij@oracle.com>
Reviewed-by: Yasufumi Kinoshita <yasufumi.kinoshita@oracle.com>