RDMA server should load authentication keys once at startup

[jti-lanl]

... instead of accessing the file for every request.

Probably just have a vector of AWSContexts, and look-through them for the user-name provided in the auth request. As a quick-and-dirty first-cut, we could at least load the default/expected user, and lookup others.

Provide a simple interface to do the lookups in the cached info. This would then be used by socket_fserver, fuse, pftool, etc.

[jti-lanl]

Okay, wait. The DAL already does this. We just need to do it on the server.