bump http client's MaxIdleConns and MaxIdleConnsPerHost

The idea here is for Pilosa to behave better under high query load where a node
might be making many connections to the other nodes in the cluster in order to
support lots of concurrent batches of SetBit queries (for example). By allowing
for more idle connections and more idle connections per host, we reduce
connection churn, and allow more connections to be reused rather than creating
new ones and potentially having many stale sockets in the TIME_WAIT state. See
golang/go#16012

executor.go

@@ -20,6 +20,7 @@ import (
 	"errors"
 	"fmt"
 	"io/ioutil"
+	"net"
 	"net/http"
 	"net/url"
 	"sort"
@@ -57,7 +58,21 @@ type Executor struct {
 // NewExecutor returns a new instance of Executor.
 func NewExecutor() *Executor {
 	return &Executor{
-		HTTPClient: http.DefaultClient,
+		HTTPClient: &http.Client{
+			Transport: &http.Transport{
+				Proxy: http.ProxyFromEnvironment,
+				DialContext: (&net.Dialer{
+					Timeout:   30 * time.Second,
+					KeepAlive: 30 * time.Second,
+					DualStack: true,
+				}).DialContext,
+				MaxIdleConns:          1000,
+				MaxIdleConnsPerHost:   200,
+				IdleConnTimeout:       90 * time.Second,
+				TLSHandshakeTimeout:   10 * time.Second,
+				ExpectContinueTimeout: 1 * time.Second,
+			},
+		},
 	}
 }