accounts: add SQL store implementation

In this commit, we add the SQLStore type which implements the
accounts.Store interface. To demonstrate that it works as expected, we
also plug this implementation into all the account unit tests to show
that they pass against the sqlite and postgres backends.

One can use `make unit pkg=accounts tags=test_db_postgres` or
`make unit pkg=accounts tags=test_db_sqlite` to test locally.

Note that 2 small timestamp related changes are made to the unit tests.
This is to compensate for timestamp precision in postgres.

accounts/interface.go

@@ -1,7 +1,9 @@
 package accounts
 
 import (
+	"bytes"
 	"context"
+	"encoding/binary"
 	"encoding/hex"
 	"errors"
 	"fmt"
@@ -55,6 +57,31 @@ func ParseAccountID(idStr string) (*AccountID, error) {
 	return &id, nil
 }
 
+// ToInt64 converts an AccountID to its int64 representation.
+func (a AccountID) ToInt64() (int64, error) {
+	var value int64
+	buf := bytes.NewReader(a[:])
+	if err := binary.Read(buf, byteOrder, &value); err != nil {
+		return 0, err
+	}
+
+	return value, nil
+}
+
+// AccountIDFromInt64 converts an int64 to an AccountID.
+func AccountIDFromInt64(value int64) (AccountID, error) {
+	var (
+		a   = AccountID{}
+		buf = new(bytes.Buffer)
+	)
+	if err := binary.Write(buf, binary.BigEndian, value); err != nil {
+		return a, err
+	}
+	copy(a[:], buf.Bytes())
+
+	return a, nil
+}
+
 // String returns the string representation of the AccountID.
 func (a AccountID) String() string {
 	return hex.EncodeToString(a[:])