Fix data race in TestLogEvent

Found with -race. Improve locking on Server.

```
==================
WARNING: DATA RACE
Write by goroutine 61:
  github.com/dotcloud/docker.(*Server).LogEvent()
      /go/src/github.com/dotcloud/docker/server.go:1846 +0x262
  github.com/dotcloud/docker.func·059()
      /go/src/github.com/dotcloud/docker/server_unit_test.go:80 +0x105

Previous read by goroutine 63:
  github.com/dotcloud/docker.func·060()
      /go/src/github.com/dotcloud/docker/server_unit_test.go:86 +0x116
  github.com/dotcloud/docker.func·062()
      /go/src/github.com/dotcloud/docker/server_unit_test.go:103 +0x4e

Goroutine 61 (running) created at:
  github.com/dotcloud/docker.TestLogEvent()
      /go/src/github.com/dotcloud/docker/server_unit_test.go:81 +0x49a
  testing.tRunner()
      /usr/local/go/src/pkg/testing/testing.go:391 +0x10f

Goroutine 63 (running) created at:
  github.com/dotcloud/docker.setTimeout()
      /go/src/github.com/dotcloud/docker/server_unit_test.go:105 +0x191
  github.com/dotcloud/docker.TestLogEvent()
      /go/src/github.com/dotcloud/docker/server_unit_test.go:90 +0x55a
  testing.tRunner()
      /usr/local/go/src/pkg/testing/testing.go:391 +0x10f
==================
--- PASS: TestLogEvent (0.40 seconds)
=== RUN TestSortUniquePorts
--- PASS: TestSortUniquePorts (0.00 seconds)
=== RUN TestSortSamePortWithDifferentProto
--- PASS: TestSortSamePortWithDifferentProto (0.00 seconds)
=== RUN TestLookupImage
--- PASS: TestLookupImage (0.02 seconds)
PASS
Found 1 data race(s)
```