Pull request: increase MaxConnsPerHost, fix #278

Merge in DNS/dnsproxy from fix-278 to master

Squashed commit of the following:

commit 19e0286
Author: Andrey Meshkov <am@adguard.com>
Date:   Sun Dec 4 20:17:01 2022 +0300

    increase MaxConnsPerHost, fix #278

upstream/upstream_doh.go

@@ -32,12 +32,13 @@ const (
 	transportDefaultIdleConnTimeout = 5 * time.Minute
 
 	// dohMaxConnsPerHost controls the maximum number of connections for
-	// each host.
-	dohMaxConnsPerHost = 1
+	// each host.  Note, that setting it to 1 may cause issues with Go's http
+	// implementation, see https://github.com/AdguardTeam/dnsproxy/issues/278.
+	dohMaxConnsPerHost = 2
 
 	// dohMaxIdleConns controls the maximum number of connections being idle
 	// at the same time.
-	dohMaxIdleConns = 1
+	dohMaxIdleConns = 2
 )
 
 // dnsOverHTTPS is a struct that implements the Upstream interface for the