Get rid of `mut` modifier in the `index::push` function

[ikopylov]

This can be done by wrapping the internal in-memory index with sync RwLock. This way we can avoid write lock acquisition at all on write path, which should improve performance in concurrent read/write scenarios. Inner RwLock will be taken only for a short period of time (much shorter than RwLock on Blob)

https://github.com/qoollo/pearl/blob/master/src/blob/index/core.rs#L259
https://github.com/qoollo/pearl/blob/master/src/blob/core.rs#L283