Add client connection check during the execution of the query

This patch adds verification of the connection with the client during 
the execution of the SQL query. The feature enables using the GUC 
variable ‘client_connection_check_interval’. The default check interval 
is 1 second. If you set the value of ‘client_connection_check_interval’ 
to 0, then the check will not be performed.
The feature will be useful in cases when, during the execution of a very 
long query, the client suddenly terminates the connection - this will 
allow backend to cancel further execution of the query and free server 
resources.