perf(dbless): load declarative schema during init() (#10932) [backport]

[flrgh]

This is a backport of #10932.

summary

With this change we are no longer reloading the declarative schema for every POST /config request (something that has been the case since 3.0, I think).

The schema load operation adds 150-200ms of latency to each request in my local testing:

# serialize requests (otherwise we're just fighting lock contention)
$ wrk -c 1 -t 1 -s wrk.lua --latency -d 60 http://localhost:8001/config

wrk.lua

local cjson = require "cjson"

local conf = {
  _transform = false,
  _format_version = "3.0",
  services = {},
}

for i = 1, 500 do
  conf.services[i] = {
    name     = "my-service-" .. tostring(i),
    protocol = "http",
    host     = "127.0.0.1",
    port     = 80,
    routes   = {
      {
        name = "my-service-route-" .. tostring(i),
        hosts = { "my-service-route-" .. tostring(i) .. ".test" },
        paths = { "/" },
      }
    }
  }
end

local body = cjson.encode(conf)

local headers = {
  ["content-type"] = "application/json",
}

local wrk = assert(_G.wrk)

wrk.scheme = "http"
wrk.host = "127.0.0.1"
wrk.port = 8001
wrk.method = "POST"
wrk.path = "/config"
wrk.body = body
wrk.headers = headers

Results for cc3b056:

  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   361.96ms   53.54ms 439.67ms   55.15%
    Req/Sec     2.41      0.57     5.00     60.00%
  Latency Distribution
     50%  391.33ms
     75%  406.90ms
     90%  416.41ms
     99%  433.45ms
  165 requests in 1.00m, 59.94MB read
Requests/sec:      2.75
Transfer/sec:      1.00MB

Results for c339a8c:

  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   160.62ms   27.79ms 229.26ms   78.06%
    Req/Sec     6.60      2.50    10.00     59.26%
  Latency Distribution
     50%  146.25ms
     75%  171.50ms
     90%  210.79ms
     99%  221.30ms
  351 requests in 1.00m, 127.51MB read
Requests/sec:      5.84
Transfer/sec:      2.12MB

why backport?

For an API-centric product, something that decreases the latency on a heavily-used API endpoint by triple digit milliseconds is worth a backport, especially given 3.3 is destined to be the next LTS.