add _setCollateralPaused()

[beepidibop]

Summary:
Comptroller.sol: added _setCollateralPaused() and changed getHypotheticalAccountLiquidityInternal() to reflect new error
ErrorReporter.sol: added new COLLATERAL_PAUSED error for the comptroller
ComptrollerStorage.sol: added V6 to store collateralGuardianPaused state

[beepidibop]

Just realized the issue required the use of checkMembership but I didn't use it. h/t @EHermstad

If checkMembership is to be used, it'll just need to be changed to public and line 727 will need to be changed to
if (collateralGuardianPaused[address(asset)] == true && borrowAmount > 0 && vars.cTokenBalance > 0 && checkMembership(account,asset)){

IMO this will only waste gas because why checkMembership again when getHypotheticalAccountLiquidityInternal is already passing markets that the user is in. This also introduces a change from external to public for checkMembership, which wastes gas for external callers who just want to check if someone's in a market.

[nourharidy]

LGTM. The purpose of the issue was to call checkMembership() for each paused collateral in the borrowAllowed hook. But this is actually a much more gas efficient and cleaner approach. Well done.