refactor: change namespace 'dsn' to 'pegasus' WIP

[acelyc111]

#1053

After the rdsn submodule has been merged into the Pegasus codebase, the previous namespace dsn is still distributed everywhere, they are some type of duplicated, useless and confused.

This patch uses namespace pegasus instead of dsn without any functionality changes, in addition there are some other very little changes, including:

• Move some FLAGS into the global namesapce, then we can use the FLAGS in any namespace without a namespace prefix.
• Rename some variables which are also type names. e.g. rename message_header.gpid to message_header.gpid_, rename some local variables from gpid to pid, and etc.
• Remove function random32 and use utils::next32() instead.

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (master@ee03ffe). Click here to learn what that means.
The diff coverage is n/a.

❗ Current head 7125557 differs from pull request most recent head 1ecb810. Consider uploading reports for the commit 1ecb810 to get more accurate results

@@            Coverage Diff            @@
##             master    #1472   +/-   ##
=========================================
  Coverage          ?   53.58%           
=========================================
  Files             ?       27           
  Lines             ?     2538           
  Branches          ?        0           
=========================================
  Hits              ?     1360           
  Misses            ?     1132           
  Partials          ?       46           

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more