[resultdb] Simplify decompression code
[resultdb] Simplify decompression code

Reuse more code and reuse buffer.
Also fix a couple of nits.

Bug: 1112127
Change-Id: I4da228f6dd6c3546d33de2bbe371dadd7525b793
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2590712
Auto-Submit: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Commit-Queue: Chan Li <332b6c56634d0f63001046c664a2370006a02420@chromium.org>
Reviewed-by: Chan Li <332b6c56634d0f63001046c664a2370006a02420@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go