Clean up the ByteBuffer utility methods

This patch was authored and released by @gwynne.

• Replace the readNullTerminatedString() implementation with the much more efficient version used by PostgresNIO.
• Made writeNullTerminatedString(), writeLengthEncodedInteger(), and writeLengthEncodedSlice() return the number of bytes written, to match ByteBuffer method conventions.
• Simplified [read|write]LengthEncodedInteger(), might be slightly faster but probably not enough to notice.
• Remove a couple of unused utility methods altogether.