fix: check backoff when join

[twoeths]

Motivation

• To conform to the gossipsub spec v1.1

Description

• add some slack time to the backoff expiration https://github.com/libp2p/specs/pull/289
• do not send GRAFT to any backoff peer, otherwise we'll immediately receive PRUNE and we'll add more backoff time to that peer

Closes #441

[codecov-commenter]

Codecov Report

Patch coverage: 60.00% and no project coverage change.

Comparison is base (cbdae04) 82.51% compared to head (956b732) 82.52%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #444 +/- ## ======================================= Coverage 82.51% 82.52% ======================================= Files 48 48 Lines 11966 11971 +5 Branches 1279 1279 ======================================= + Hits 9874 9879 +5 Misses 2092 2092 ``` | [Impacted Files](https://app.codecov.io/gh/ChainSafe/js-libp2p-gossipsub/pull/444?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ChainSafe) | Coverage Δ | | |---|---|---| | [src/index.ts](https://app.codecov.io/gh/ChainSafe/js-libp2p-gossipsub/pull/444?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ChainSafe#diff-c3JjL2luZGV4LnRz) | `70.12% <42.85%> (+0.02%)` | :arrow_up: | | [src/constants.ts](https://app.codecov.io/gh/ChainSafe/js-libp2p-gossipsub/pull/444?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ChainSafe#diff-c3JjL2NvbnN0YW50cy50cw==) | `100.00% <100.00%> (ø)` | |

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