If client code initiates an FDB operation to a TLS cluster, and then
immediately exits the main thread, then OpenSSL's atexit handler would
potentially run while the network thread is attempting to do TLS
operations, and thus crash.
This commit removes the OpenSSL atexit hander, and instead relies on a
client intentionally ending the network thread to do TLS cleanup. If
the client code exits without stopping the network thread, then we'll
never free OpenSSL data structures, which is the safer thing to do.
This requires the certificate chain to load successfully, otherwise
fdbcli will error out at an earlier point due to Net2 not being able to
configure TLS.
I went back and dug through all of the "what functions can throw what
types", and made sane decisions about them. boost errors are
aggressively translated into FDB ones, whcih might result in multiple
lines of logging about errors, but this is in infrequently run code, so
it should be fine.
The idea being that we keep around a TLSConfig that the configuration
that the user has provided, and then when we want to intialize an SSL
context, we ask the TLSConfig to load all certificates and return us a
LoadedTLSConfig that is a concrete set of certificate bytes in memory.
initTLS now just takes the in-memory bytes and applies them to the ssl
context.
This is a large refactor to lead up into certificate refeshing, where we
will periodically check for changes to the certificates, and then
re-load them and apply them to a new SSL context.
