[SE,XLA] Switch to using multiple streams in xla_device_context

Instead of having one stream for compute, host-to-device and device-to-host transfers, switch to having separate streams, just like the GPU does.
Add a se::Event field to XlaTensor to allow accurate inter-stream dependencies to be created.

As part of this:
 - Fix TransferManager::TransferLiteralFrom/ToDevice to correctly make generated substreams wait on their master stream.
 - Fix Stream::BlockHostUntilDone() to not block on or return substreams. This behavior is completely broken and not only nondeterministically returns substreams to the pool but causes indefinite hangs with the HostStream.

PiperOrigin-RevId: 203726543