feat: use BufWriter on NippyJarWriter

[joshieDo]

As the title suggests. This change speeds up the writing during historical sync because it reduces the number of write system calls. By doing so, we achieve a writing speed that is comparable to or better than that of the main branch.

However, adopting this change means that whenever we work with File::metadata.len(), File::set_len or handle seeks/position streams, we need to flush it first

[shekhirin]

LGTM, good find. File doesn't use any buffering on writing underneath, and goes directly to libc::write.

[mattsse]

lgtm