[WIP] use getLong when matching byte sequences in ByteString

[pjfanning]

• see Revisit indexOfSlice performance improvements #2321
• there are more changes that could be attempted based on what is in Netty ByteBufUtil but we've also seen getLong helping quite a bit with indexOf
• aim is to benchmark this and if it helps, get it merged before possibly trying further Netty ByteBufUtil approaches
  https://github.com/netty/netty/blob/a5343227b10456ec889a3fdc5fa4246f036a216d/buffer/src/main/java/io/netty/buffer/ByteBufUtil.java#L242
• the CI issue is due to a package private method causing a mima issue - it can be easily filtered later