Make module more thread-safe.

To prevent multiple threads from using the same connection at the same
time, put the connection into thread local storage.

Synchronize setcontents, makedir and removedir calls.
Replace synchronization with separate connections.

Using separate connections from pysmb should be sufficient to provide
thread safety, so there is no need to synchronize individual methods.

Close connections explicitly in SMBFS.close().
Change mime type to virtual/smb.

virtual/filehub is not descriptive of the type of filesystem this module
represents, so use one similar to fs-dropbox.
Use @synchronize for all calls

The use of thread local storage was not enough to prevent some odd, yet
infrequent, errors from the SMB server from appearing during threaded
unit tests.  Locking all exposed calls works, so use that means instead
of TLS.
Clarify DeletePendingError message.