Make the max queue size and give up time configurable

Today, the maximum number of items in the queue, as well as how long we
keep attempting to send each item, is hard-coded and not changed by end
users.

While they are totally adequate for chasquid's main use cases, it can
still be useful for some users to change them.

So this patch adds two new configuration options for those settings.
They're marked experimental for now, so we can adjust them if needed
after they get more exposure.

Thanks to Lewis Ross-Jones <lewis_r_j@hotmail.com> for suggesting this
improvement, and help with testing it.

internal/config/config.go

@@ -7,6 +7,7 @@ package config
 import (
 	"fmt"
 	"os"
+	"time"
 
 	"blitiri.com.ar/go/log"
 
@@ -30,6 +31,9 @@ var defaultConfig = &Config{
 	DropCharacters:   proto.String("."),
 
 	MailLogPath: "<syslog>",
+
+	MaxQueueItems:   200,
+	GiveUpSendAfter: "20h",
 }
 
 // Load the config from the given file, with the given overrides.
@@ -67,6 +71,12 @@ func Load(path, overrides string) (*Config, error) {
 		}
 	}
 
+	// Validate the GiveUpSendAfter value.
+	if _, err := time.ParseDuration(c.GiveUpSendAfter); err != nil {
+		return nil, fmt.Errorf(
+			"invalid give_up_send_after value %q: %v", c.GiveUpSendAfter, err)
+	}
+
 	return c, nil
 }
 
@@ -126,6 +136,13 @@ func override(c, o *Config) {
 	if o.HaproxyIncoming {
 		c.HaproxyIncoming = true
 	}
+
+	if o.MaxQueueItems > 0 {
+		c.MaxQueueItems = o.MaxQueueItems
+	}
+	if o.GiveUpSendAfter != "" {
+		c.GiveUpSendAfter = o.GiveUpSendAfter
+	}
 }
 
 // LogConfig logs the given configuration, in a human-friendly way.
@@ -153,4 +170,13 @@ func LogConfig(c *Config) {
 	log.Infof("  Dovecot auth: %v (%q, %q)",
 		c.DovecotAuth, c.DovecotUserdbPath, c.DovecotClientPath)
 	log.Infof("  HAProxy incoming: %v", c.HaproxyIncoming)
+	log.Infof("  Max queue items: %d", c.MaxQueueItems)
+	log.Infof("  Give up send after: %s", c.GiveUpSendAfterDuration())
+}
+
+func (c *Config) GiveUpSendAfterDuration() time.Duration {
+	// We validate the string value at config load time, so we know it is well
+	// formed.
+	d, _ := time.ParseDuration(c.GiveUpSendAfter)
+	return d
 }