[#316] locode/airports: Scan csv table into memory

Scanning csv-table entries one-by-one takes significant time and system
resources. To speed up random access to table records, on the first call,
the table is pumped into memory (map). On subsequent calls, I/O operations
are not performed.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/util/locode/db/airports/db.go

@@ -3,6 +3,7 @@ package airportsdb
 import (
 	"fmt"
 	"os"
+	"sync"
 )
 
 // Prm groups the required parameters of the DB's constructor.
@@ -30,6 +31,12 @@ type Prm struct {
 // The DB is immediately ready to work through API.
 type DB struct {
 	airports, countries pathMode
+
+	airportsOnce, countriesOnce sync.Once
+
+	mCountries map[string]string
+
+	mAirports map[string][]record
 }
 
 type pathMode struct {