Sending large packets is slow (on the client)

[Jasper-Bekkers]

Bug Report

I've only made a minor modification to the helloworld sample so the message takes bytes instead of a string. Then I send it around 2G of data in a single packet. Tonic seems to spend a significant time (much more then would be the norm for a normal 2GB memcpy) moving this data around.

Ideally it should just zero-copy / move the data into the OS.

Minimal repro attached

tonic-repro.zip

Relates to #352 as well

[Jasper-Bekkers]

Both this and #352 share the same repro case btw.

[LucioFranco]

also @seanmonstar :)