IFRT proxy: asynchronous and faster MakeArrayFromHostBuffer

[copybara-service]

IFRT proxy: asynchronous and faster MakeArrayFromHostBuffer

Note: I use the term 'control path' to refer to everything except HostBufferStore.Store() and HostBufferStore.Lookup() operations.

This CL improves performance with the following changes:

• The client manufactures array handles for MakeArrayFromHostBufferRequest (instead of the server generating them) and returns to the caller immediately after sending the request. Since ordering is maintained for control path requests across the proxy, future operations on the array do not require any special handling.
• The data-path HostBufferStoreRequest that corresponds to a MakeArrayFromHostBufferRequest is not ordered by the client (before or after) the MakeArrayFromHostBufferRequest. On the server-side, the loop that handles control path requests, when it sees a MakeArrayFromHostBufferRequest, blocks until the corresponding HostBufferStoreRequest is processed.
• The data-path (HostBufferStore implementation) and control path now use different gRPC channels.
• Resulting performance: BM_HostToDeviceAsync/1M/2k results in more than 3 GB/s, making it bottlenecked by gRPC stream.Write() latency. BM_HostToDeviceAsync/1K/98k (~4MB/s) was already bottlenecked by gRPC stream.Write() latency.

This CL also:

• Adds more XProf tracemes
• Introduces global_flags.h and global_flags_google.cc so we can conveniently use command-line flags in the proxy-client. This may not be ideal from a clean-code perspective, but makes it much easier to develop and debug the client.