Since nfs_stat_async is deprecated.

Required according to RFC 3986:

> An implementation should accept uppercase letters as equivalent to
> lowercase in scheme names

Closes #330

Fixes two build failures with libnfs on Windows.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Eliminate dependency on io_thread_get().

The Connect method can be called between Schedule and lock. In that case, when
locked, the state is already set to CONNECTING of READY and the condition won't
be signaled anymore.

Return false on error, not nullptr.

Fixes bogus assertion failure.

Share the NFS connection with the NFS input plugin.

Read all directory entries into memory and close the struct nfsdir
before returning the StorageDirectoryReader instance.  This is what
libnfs does, anyway.

Creating a NfsStorage sets its own export_name as the "base".  Now
NfsFileReader can use this information to derive the export_name to be
mounted, instead of guessing.  This solves the "too many connection"
problem on the NFS server while updating the database.

"override" implies "virtual".

Assume the configured filesystem character set is also used by the NFS
server.

Swap the order of adding the URI and the slash, because appending is
cheaper than inserting.