OPENDJ-1802 Make ByteStringBuilder methods more intentional

ByteStringBuilder.java:
Changed:
- append(byte) to appendByte(int)
- append(short) to appendShort(int)
- append(int) to appendInt(int)
- append(long) to appendLong(long)
- append(Object) to appendObject(Object)
- append(String) to appendUtf8(String)
- append(char[]) to appendUtf8(char[])
- All remaining append() to appendBytes().
I changed appendByte() and appendShort() to accept int parameters.
This avoids continuously casting, at the expense of a possible loss of the high bits.