Pull request: * all: move internal Go packages to internal/

Merge in DNS/adguard-home from 2234-move-to-internal to master

Squashed commit of the following:

commit d26a288cabeac86f9483fab307677b1027c78524
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Oct 30 12:44:18 2020 +0300

    * all: move internal Go packages to internal/

    Closes #2234.

internal/stats/stats.go

@@ -0,0 +1,85 @@
+// Module for managing statistics for DNS filtering server
+
+package stats
+
+import (
+	"net"
+	"net/http"
+)
+
+type unitIDCallback func() uint32
+
+// DiskConfig - configuration settings that are stored on disk
+type DiskConfig struct {
+	Interval uint32 `yaml:"statistics_interval"` // time interval for statistics (in days)
+}
+
+// Config - module configuration
+type Config struct {
+	Filename          string         // database file name
+	LimitDays         uint32         // time limit (in days)
+	UnitID            unitIDCallback // user function to get the current unit ID.  If nil, the current time hour is used.
+	AnonymizeClientIP bool           // anonymize clients' IP addresses
+
+	// Called when the configuration is changed by HTTP request
+	ConfigModified func()
+
+	// Register an HTTP handler
+	HTTPRegister func(string, string, func(http.ResponseWriter, *http.Request))
+
+	limit uint32 // maximum time we need to keep data for (in hours)
+}
+
+// New - create object
+func New(conf Config) (Stats, error) {
+	return createObject(conf)
+}
+
+// Stats - main interface
+type Stats interface {
+	Start()
+
+	// Close object.
+	// This function is not thread safe
+	//  (can't be called in parallel with any other function of this interface).
+	Close()
+
+	// Update counters
+	Update(e Entry)
+
+	// Get IP addresses of the clients with the most number of requests
+	GetTopClientsIP(limit uint) []string
+
+	// WriteDiskConfig - write configuration
+	WriteDiskConfig(dc *DiskConfig)
+}
+
+// TimeUnit - time unit
+type TimeUnit int
+
+// Supported time units
+const (
+	Hours TimeUnit = iota
+	Days
+)
+
+// Result of DNS request processing
+type Result int
+
+// Supported result values
+const (
+	RNotFiltered Result = iota + 1
+	RFiltered
+	RSafeBrowsing
+	RSafeSearch
+	RParental
+	rLast
+)
+
+// Entry - data to add
+type Entry struct {
+	Domain string
+	Client net.IP
+	Result Result
+	Time   uint32 // processing time (msec)
+}