[SPARK-17876] Write StructuredStreaming WAL to a stream instead of materializing all at once

## What changes were proposed in this pull request?

The CompactibleFileStreamLog materializes the whole metadata log in memory as a String. This can cause issues when there are lots of files that are being committed, especially during a compaction batch.
You may come across stacktraces that look like:
```
java.lang.OutOfMemoryError: Requested array size exceeds VM limit
at java.lang.StringCoding.encode(StringCoding.java:350)
at java.lang.String.getBytes(String.java:941)
at org.apache.spark.sql.execution.streaming.FileStreamSinkLog.serialize(FileStreamSinkLog.scala:127)

```
The safer way is to write to an output stream so that we don't have to materialize a huge string.

## How was this patch tested?

Existing unit tests

Author: Burak Yavuz <brkyvz@gmail.com>

Closes #15437 from brkyvz/ser-to-stream.