Remove valkey from filenames and update header guards

[bjosv]

Since we have change the file structure for public headers we can remove valkey from their filenames.
As example, we will have #include <valkey/ssl.h> instead of #include <valkey/valkey_ssl.h>.

• valkeycluster.h -> cluster.h
• valkeycluster_ssl.h -> cluster_ssl.h
• valkey_rdma.h -> rdma.h
• valkey_ssl.h -> ssl.h

This PR also includes:

• The header guards are updated to follow the common rule: <PROJ>_<PATH>_<FILENAME>_H
• Renaming of 2 cluster source files, removing valkey from their names.
• Make sure ssl.h can be included before valkey.h (required by clang-format after the rename).