Fix http2_keepalive panics on the client side

[AmeliasCode]

Motivation

#1726 fixed http2_keepalive on the server side, but panics were still present when using http2_keepalive on the client side.

Solution

Add the necessary timer to the SharedExec when constructing it via tonic::transport::service::Connection.

[akiradeveloper]

@AmeliasCode My test now works using the code on your repository. Thank you.

$ cargo run 1
   Compiling tonic-build v0.11.0 (https://github.com/AmeliasCode/tonic#91a189d6)
   Compiling tonic v0.11.0 (https://github.com/AmeliasCode/tonic#91a189d6)
   Compiling tonic-hyper10-debug v0.1.0 (/Users/akira/Source/tonic-hyper10-debug)

[djc]

Great, thanks!