Create an npm module for message cache

[Mikerah]

When the message cache implementation is stable and fully tests (e.g when #3 is complete), it makes sense to refactor it into its own npm module. Several of these data structures that were created for libp2p such as the time cache have their own modules and are widely used for different projects.

[GregTheGreek]

Is this relevant?

[Mikerah]

Maybe not. But, the message cache is it's own data structure just like the time cache is. The time cache is used throughout js-libp2p projects. I'm not sure if the message cache is as extensively used.

[vasco-santos]

I would suggest that if we create a new module that needs a message cache, we extract it from here to its own module. Until then, we can close this issue