WL#1703: Make bin_log_cache_size session variable

This variable is currently implemented as GLOBAL only, while logically it is 
session variable as it is corresponds to per connection buffer. 

So We should change it to be SESSION variable instead.

There is also practical reason for that - one can increase it for batch jobs
having big transactions to eliminate for disk usage in this case. 

This should be small task which can be grabbed when someone has few extra hours 
:)