stream-ssl: Make no-op reconfiguration cheap.

stream-ssl: Make no-op reconfiguration cheap.

Until now, the stream_ssl functions for configuring private keys,
certificates, and CA certificates have always called into OpenSSL to read
a file.  This commit instead makes them do that only if the file name
changed (or it has been 60 seconds since we last tried, in case someone
installed the file behind our backs).

This allows us to factor some code out of vswitchd.  In an upcoming commit
we will want to do essentially the same thing from ovsdb-server, so this
avoid code redundancy.