Increase max value for max_prepared_stmt_count from 1 to 4 million
(no change to the default value of 16K)

Motivation: We want to be able to test 40K connections using our standard usage
of Sysbench (with prepared statements and up to 8 tables).

Sysbench with prepared statements and 8 tables gives 11 queries x 8 tables = 88
prepared statements per connection.
Then 88 x 40K connections = 3520K prepared statements

Change-Id: I478dc32750c9aef10334fbe2e796f8ffa90ef590