btl/uct: update btl default limits

[hjelmn]

When a module is created for a UCT transport we take the min of the transport limits and whatever the default is in the module. This is done to support user provided limits. The issue is the limits are too low and they end up overriding the transport limits when the user hasn't specified anything. To fix this issue this commit changes the put/get limit to SIZE_T_MAX and the send limits to 256kiB.

This issue was detected with a large message benchmark where it was discovered that ob1 was not performing put or get operations over 4MiB even though the hardware supported larger sizes.