fix #2027

fix #2083.

By looking at name like `apisix_bandwidth` or `apisix_http_latency`,
it is hard to guess their units. Also, it is not so clear for where does
the overhead come from for `apisix_http_overhead` at the first glance.

* feature: divide config.yaml into two files config-custom.yaml & config-default.yaml .

fix #1923 

fix #2058

* fixed test

* using a new metric `node_info` to store the hostname

Fix #2064.

related #1877

fix #2022

fix #2039

See https://fetch.spec.whatwg.org/#cors-protocol-and-http-caches.

fix #2019

bugfix: comparing etcd key's modifiedIndex as a number may cause an error when access to prometheus exporter api (#2035)

fix #2025

FIX #2037

Fix #1718

* Added test case for token intropsection with Keycloak identity server
* Renamed oauth doc to openid-connect to correctly link with the plugin name

* feat: allow customizing worker_processes

Close #2016

be called multiple times.

fix #1986

fix #2008

fix #2010

Fix #1855

fix #1983

Removed multiple "incubator-" prefixed from urls in both EN and CN
getting-started documents

Updated the cd command in the getting started steps so that it moved
into the correct directory after cloning

fix #1872, which not using the right way to install from luarocks.

* bugfix: passed the copied `api_ctx`, it may be freed if the `api_ctx`
  was called in back timer async.

Fixed #1935.

Supporting distribution and execution scripts, we can implement many new features, 
such as plug-in orchestration.

fixed #1987

* bugfix: fail to handshake if there is no certificate matched by host.
* fix:  grpc proxy test case

fix #1382

* perf: no longer generate unnecessary nginx conf for better performance.
* benchmark: sync nginx.conf for fake-apisix.

> Is this PR backward compatible?

Disable two plugins by default(proxy-cache, proxy-mirror), if the user wants to enable them, need to modify the conf/config.yaml by manual.