Fix compile errors in the server and client.

[husafan]

The client needs to define the type of struct since instances were already allocated in the definition of the grpc_metadata array. Also, the default nullptr's can be removed from the nested obfuscated struct's creation in the same statement.

The server needs to undef the Free macro defined by Rcpp so it can be defined by absl low_level_alloc later.

These changes were tested against grpc version 1.41.1 (12a4a6f8c06b96e38f8576ded76d0b79bce13efd7560ed22134c2f433bc496ad) on Ubuntu 21.04 with gcc (Ubuntu 10.3.0-1ubuntu1) 10.3.0.

Fixes #37

[nfultz]

Can you add to your post which version this fix is for? eg output of grpc_version() - we will need to eventually update the metadata / documentation about dependencies.

[husafan]

Done. I've added the grpc, Ubuntu and gcc versions. Please feel free to not merge this if it doesn't repro. I have it working in my project now, so no rush. Thanks!