MDEV-15019 - InnoDB: store ReadView on trx

This will allow us to reduce critical section protected by
trx_sys.mutex:
- no need to maintain global m_free list
- eliminate if (trx->read_view == NULL) condition.

On x86_64 sizeof(Readview) is 144 mostly due to padding, sizeof(trx_t)
with ReadView is 1200.

Also don't close ReadView for read-write transactions, just mark it
closed similarly to read-only.

Clean-up: removed n_prepared_recovered_trx and n_prepared_trx, which
accidentally re-appeared after some rebase.

storage/innobase/include/trx0trx.ic

@@ -211,17 +211,6 @@ ok:
 	trx->dict_operation = op;
 }
 
-/**
-@param trx		Get the active view for this transaction, if one exists
-@return the transaction's read view or NULL if one not assigned. */
-UNIV_INLINE
-ReadView*
-trx_get_read_view(
-	trx_t*		trx)
-{
-	return(!MVCC::is_view_active(trx->read_view) ? NULL : trx->read_view);
-}
-
 /**
 @param[in] trx		Transaction to check
 @return true if the transaction is a high priority transaction.*/