coreunix: addFile: encapsulate `FlushMemFree` call

The `addFile` functions is pretty much the entry point for anyone looking to understand what is going on when calling `ipfs add` so it would be useful to keep it as clean as possible, I think the flush logic added in https://github.com/ipfs/go-ipfs/pull/3888 could be encapsulated in a separate function.

https://github.com/ipfs/go-ipfs/blob/7853e53860805e08a212d78c4baa5d59bff99ba8/core/coreunix/add.go#L421-L439

(Also we could make `liveCacheSize` a `const` and document its use and the value chosen.)

Start in core/coreunix/add.go at the linked addFile code and inspect the existing FlushMemFree logic. Extract that logic into a separate function, then consider the requested liveCacheSize constant and documentation; done when addFile is cleaner and the existing coreunix checks still pass.

Written by the indexing model from the issue text.