InnoDB transactions are not aligned at cache lines

When the lock-free hash table trx_sys.rw_trx_hash was implemented in MariaDB Server 10.3 was implemented, the cache line alignment of transaction objects became more important when iterating the hash table. All trx_t are allocated via trx_pools, and it is easy to change that to return cache line aligned objects.

Furthermore, rw_trx_hash_element_t::mutex is unnecessarily large. Replacing it with the futex (or Microsoft Windows SRWLOCK) based srw_mutex will shrink the size of a hash table element to 64 bytes (the size of a cache line on most AMD64 implementations). There was an internal interface for PERFORMANCE_SCHEMA instrumentation for the element mutexes, but it was not exposed via all_innodb_mutexes.