chore: use asf license header

[neverchanje]

What problem does this PR solve?

This PR I licensed every file in pegasus with Apache license 2 and gave credit to the apache software foundation.
The licensing policy follows https://www.apache.org/legal/src-headers.html.

I used the license tool https://creadur.apache.org/rat/apache-rat/ which is officially recommended by the ASF. Every ASF project uses this tool to validate a release. About how to use this tool, I've written a wiki here: https://cwiki.apache.org/confluence/display/PEGASUS/Validating+a+staged+release.

> java -jar apache-rat-0.13.jar -E ./.rat-excludes -d . -f -a

Ignored 0 lines in your exclusion files as comments or empty lines.

*****************************************************
Summary
-------
Generated at: 2020-10-30T08:17:31+08:00

Notes: 3
Binaries: 0
Archives: 0
Standards: 249

Apache Licensed: 249
Generated Documents: 0

JavaDocs are generated, thus a license header is optional.
Generated files do not require license headers.

0 Unknown Licenses

"0 Unknown Licenses" means no incorrectly licensed file. For some files that are excluded for license checking, they've been collected in the .rat-excludes file. Configurations, documents are not licensed (for now).

➜  incubator-pegasus git:(license) rg Xiaomi
LICENSE
232: Copyright (c) 2017-present, Xiaomi, Inc.  All rights reserved.

I've searched and confirmed there is no file distributed under "Xiaomi".

This is a large PR. I've carefully reviewed every file. Please take some time to review.