Add lock to fix data race in internal/oplog/oplog_tail.go

[timvaillancourt]

Fix for data race:

test_1    | === RUN   TestBasicApplyLog
test_1    | Dumping the oplog into "/tmp/example.bson.739782274"
test_1    | ==================
test_1    | WARNING: DATA RACE
test_1    | Write at 0x00c4202024a0 by goroutine 39:
test_1    |   github.com/percona/mongodb-backup/internal/oplog.(*OplogTail).tail()
test_1    |       /go/src/github.com/percona/mongodb-backup/internal/oplog/oplog_tail.go:149 +0x7aa
test_1    | 
test_1    | Previous read at 0x00c4202024a0 by goroutine 38:
test_1    |   github.com/percona/mongodb-backup/internal/oplog.(*OplogTail).tailQuery()
test_1    |       /go/src/github.com/percona/mongodb-backup/internal/oplog/oplog_tail.go:167 +0x1bb
test_1    |   github.com/percona/mongodb-backup/internal/oplog.(*OplogTail).tail()
test_1    |       /go/src/github.com/percona/mongodb-backup/internal/oplog/oplog_tail.go:160 +0x4cd
test_1    | 
test_1    | Goroutine 39 (running) created at:
test_1    |   github.com/percona/mongodb-backup/internal/oplog.Open()
test_1    |       /go/src/github.com/percona/mongodb-backup/internal/oplog/oplog_tail.go:50 +0xf5
test_1    |   github.com/percona/mongodb-backup/internal/oplog.TestBasicApplyLog()
test_1    |       /go/src/github.com/percona/mongodb-backup/internal/oplog/oplog_apply_test.go:29 +0x1ad
test_1    |   testing.tRunner()
test_1    |       /usr/local/go/src/testing/testing.go:777 +0x16d
test_1    | 
test_1    | Goroutine 38 (running) created at:
test_1    |   github.com/percona/mongodb-backup/internal/oplog.open()
test_1    |       /go/src/github.com/percona/mongodb-backup/internal/oplog/oplog_tail.go:85 +0x2b4
test_1    |   github.com/percona/mongodb-backup/internal/oplog.Open()
test_1    |       /go/src/github.com/percona/mongodb-backup/internal/oplog/oplog_tail.go:46 +0x54
test_1    |   github.com/percona/mongodb-backup/internal/oplog.TestBasicApplyLog()
test_1    |       /go/src/github.com/percona/mongodb-backup/internal/oplog/oplog_apply_test.go:29 +0x1ad
test_1    |   testing.tRunner()
test_1    |       /usr/local/go/src/testing/testing.go:777 +0x16d