all: add configuration to limit max concurrent requests

Currently, there's no upper bound for how many requests that ctrld will
handle at a time. This could be problem on some low capacity routers,
where CPU/RAM is very limited.

This commit adds a configuration to limit how many requests that will be
handled concurrently. The default is 256, which should works well for
most routers (the default concurrent requests of dnsmasq is 150).

docs/config.md

@@ -157,6 +157,14 @@ stale cached records (regardless of their TTLs) until upstream comes online.
 - Required: no
 - Default: false
 
+### max_concurrent_requests
+The number of concurrent requests that will be handled, must be a non-negative integer. 
+Tweaking this value depends on the capacity of your system.
+
+- Type: number
+- Required: no
+- Default: 256
+
 ## Upstream
 The `[upstream]` section specifies the DNS upstream servers that `ctrld` will forward DNS requests to.