refactor(router): atc router clean

[chronolaw]

Summary

Refactoring router's code, separate expressions/compat.

This PR should be merged after 3.0 release.

Changes:

• rename atc_compat.lua to compat.lua
• add a new file expressions.lua
• move lots of logic into atc.lua
• rename get_atc to get_expression
• rename route_priority to get_priority
• rename atc_escape_str to escape_str
• remove function paths_resort
• transplant perf(router) if header matching is not used, don't read headers #9327
• transplant tests(router) optimize unit test for router #9329
• transplant fix(router/atc) detect and return error for concurrent rebuilds durin… #9343
• transplant fix(router) old_route has wrong assignment order #9346
• transplant tests(router) add more test cases for incremental build #9357
• transplant fix(router) prefix match no longer normalize input #9389
• transplant fix(router) anchor regex paths in traditional_compatible mode #9394
• transplant fix(router): check empty route field #9451
• transplant fix(router/atc): check expression building error #9480