Name collision between the isCancelable getter and the storage variable

[PaulRBerg]

As pointed out by @andreivladbrg in #467 (comment), there is a name collision between the isCancelable getter and the isCancelable storage variable (a field in the LockupLinear.Stream and LockupDynamic.Stream struct).

• The getter returns false when the stream is SETTLED
• However, the storage variable is still true in the struct returned by the getStream getter when the stream is SETTLED

Thus, we should consider finding a new name for the isCancelable storage variable.

If we don't find a solution ourselves, we should ask our auditors for guidance.