Make FlexVotingClient governor mutable

[davidlaprade]

As we know all too well, governors change and the governor address associated with a given governance token and/or timelock can be updated over time.

For this reason, the FlexVotingClient abstract contract should reflect this. Specifically:

• the GOVERNOR variable should be mutable, not immutable
• the abstract contract should define a virtual function to update the GOVERNOR value -- we might call it updateGovernor -- which child contracts will be required to implement

This is also needed for better proxy support. There currently is no way to set the GOVERNOR value in an initializer.

This was raised here and here