OPENDJ-1307 Migrate server ASN1 classes to SDK


Code review: Matthew Swift


ByteStringBuilder.java:
Renamed clear(int capacity) to clearAndTruncate(int thresholdCapacity, int newCapacity).
Minor code cleanups.

ASN1OutputStreamWriter.java
In writeEndSequence(), moved some code to ByteStringBuilder.clearAndTruncate() + called the latter.

ByteStringBuilderTestCase.java:
Added tests for ByteStringBuilder.clearAndTruncate().
Added method "_" to ease bytes readability.