http2: core.js replace var with let

const has to be used in for in loop (https://medium.com/@mautayro/es6-variable-declaration-for-loops-why-const-works-in-a-for-in-loop-but-not-in-a-normal-a200cc5467c2)

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines