Fix spoolfile message formatting

[arnecls]

The purpose of this pull request

Since 0.5.0, the spooling format is broken, as messages are written to the spool file in plaintext and not as base64 encoded protobuf.

Config to verify

"Generator":
    Type: "consumer.Profiler"
    Streams: "profile"
    Runs: 1000000
    Batches: 1000
    Characters: "abcdefghijklmnopqrstuvwxyz .,!;:-_"
    Message: "%256s"

"SocketOut":
    Type: "producer.Socket"
    Streams: "profile"
    ChannelTimeoutMs: 10
    Address: "127.0.0.1:5880"
    Modulators: 
        - "format.Runlength"
    ConnectionBufferSizeKB: 128
    FallbackStream: "spooling"
    Batch:
        TimeoutSec: 1
    Acknowledge: "OK"

"Spooler":
    Type: "producer.Spooling"
    Streams: "spooling"
    Path: "test"
    MaxFileSizeMB: 512```

Checklist

• make test executed successfully
• integration test provided

[msiebeneicher]

NIT - Resolve blocking spooler during shutdown:

[2019-04-15 17:42:07 CEST] ERROR Timeout during onPrepareStop. PluginID=bi-spooler PluginType=producer.Spooling
[2019-04-15 17:42:07 CEST] DEBUG Executing stop command PluginID=bi-spooler PluginType=producer.Spooling
[2019-04-15 17:42:12 CEST] ERROR At least one producer found to be blocking.

[coveralls]

Coverage decreased (-0.1%) to 71.642% when pulling 2f190cf on fix-spool-format into 2081050 on master.

[coveralls]

Coverage decreased (-0.05%) to 71.711% when pulling c977963 on fix-spool-format into 2081050 on master.