read_view_open_now() and read_cursor_view_create_for_mysql() do a scan

of the list of all open transactions (trx_sys->trx_list) with the

kernel_mutex locked. On high-concurrency workloads (i.e. sysbench

read-only with number of threads >= 512) the cost of this scan is

huge.

The patch is ported from Percona Server, and contains 2 optimizations

to reduce the cost during read view creating.

1. Pre-allocate a read view structure for every transaction

No need to allocate a read view struct for each read_view_open_now()

call in a transaction, which is too expensive, allocate once and reused

it aterwards when need.

2. Maintain a trx_id array. While creating a read view, simply copy

the array without traversing the transaction rw_list.

Introduces a concept of "trx descriptors" which is a global

ordered array containing IDs of transactions in either TRX_ACTIVE or

TRX_PREPARED state. It allows to replace the trx_list scan in

read_view_open_now() and read_cursor_view_create_for_mysql() with a

binary search on the descriptors array and two memcpy()s.

The initial size of the descriptors array is 1000 slots (i.e. 8000

bytes). It is resized whenever we need more descriptors.

Since there is no transaction serialization numbers (i.e. trx->no) in the

descriptors array, this check was replaced by keeping a separate,

trx->no ordered list (trx_sys->trx_serial_list). Getting the current

minimum for the current read view is then simply a matter of getting

trx->no of the first element from trx_serial_list.