flow: get/set memcap value

flow: get/set memcap value

This adds new functions that will be called
through unix-socket and permit to update
and show memcap value.

The memcap value needs to be handled in a
thread safe way, so for this reason it is
declared as atomic var.

FlowGetMemuse() function is made as public
because the memuse value will be shown
through unix-socket.