http2: multiple cleanups and s/streamClosed/close

Includes multiple cleanups in core.js along with:

• Use correct connect event for TLSSocket
• Use 'close' event instead of streamClosed

/cc @nodejs/http2

Fixes: https://github.com/nodejs/node/issues/15303

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

http2