feat: minor improvements in the quotas system

[x] https://github.com/spearbit-audits/review-gearbox/issues/27 and https://github.com/spearbit-audits/review-gearbox/issues/28 have been fixed by preventing the Tumbler to add a quoted token with the initial rate == 0
[x] https://github.com/spearbit-audits/review-gearbox/issues/84 has been addressed by reverting when the user tries to purchase quota for token == underlying inside the CreditFacadeV3 logic. Quota for the underlying token cannot be purchased because it can't be a quoted token from the very beginning. The behavior has also been documented in the natspec documentation of the updateQuota function in the ICreditFacadeV3Multicall interface. Note: Gearbox should consider adding a direct check inside the PoolQuotaKeeper logic that prevents a IRateKeeper from adding a quoted token equal to the PKQ underlying.
[x] https://github.com/spearbit-audits/review-gearbox/issues/93 and https://github.com/spearbit-audits/review-gearbox/issues/94 have been fixed by reverting in PoolQuotaKeeper._updateQuota when a user tries to purchase quota for a token with rate == 0. When the token's rate is equal to zero, it means that the token has not been added and fully configured yet as a proper quoted token.
[x] https://github.com/spearbit-audits/review-gearbox/issues/29 has been acknowledged by Gearbox with a natspec comment in the TumblerV3 contract
[x] https://github.com/spearbit-audits/review-gearbox/issues/31 has been acknowledged by Gearbox with a natspec comment in the addQuotaToken function of the PoolQuotaKeeperV3 contract

Gearbox-protocol / core-v3

feat: minor improvements in the quotas system #274