zero out database key when done using it
zero out database key when done using it

This commit zeros out the stored password when the last instance is closed.
The password has to be stored for multi-threaded mode, since new threads
will need the password in order to access the database.

refs #3402 https://dev.guardianproject.info/issues/3402

spthaolt/libsqlfs,eighthave/libsqlfs,eighthave/libsqlfs,spthaolt/libsqlfs,eighthave/libsqlfs,guardianproject/libsqlfs,guardianproject/libsqlfs,guardianproject/libsqlfs,spthaolt/libsqlfs,spthaolt/libsqlfs,eighthave/libsqlfs,spthaolt/libsqlfs,guardianproject/libsqlfs,eighthave/libsqlfs,guardianproject/libsqlfs