overflow possible in get Imbalance - very low likliehood.

[ilanDoron]

In the getImbalance function, the multiplication totalImbalance *= resolution is performed unchecked.
Since the resolution field can be arbitrarily big and totalImbalance can reach 2

63−1, overflows are possible.

A check on writing the resolution variable could be introduced to avoid this.
Since the function is only used to retrieve information and in a benevolent case resolution should not be
this big, this is unlikely. But because this function is used in the getRate function, a consequence could be
returning a wrong rate to users.
The same applies to currentBlockImbalance *= resolution.

[manhlx3006]

Adjusted in this PR #422