Updates a workflow instance object in persistant storage.
Limitation: Context values must consist of valid Unicode characters.
NULL bytes are explicitly not allowed. Binary data storage is NOT possible.
Limitation: The maximum length of context values is 32 KByte.
Limitation: If the parameter value is undef the parameter will not be
persisted. After restoring the workflow instance from persistent storage
the corresponding entry will not exist.
Volatile Context Parameters
Context parameters starting with an underscore _ will NOT be
saved persistently in the database. You can use such parameters
for storing truly temporary data that does not need to be stored in the
database (and that will NOT survive saving and retrieving the workflow
instance from the database!) or e. g. for caching objects that can also be
retrieved from the database.
Such volatile context parameters can have arbitrary size, may contain
arbitrary Perl data structures (including Object references) or arbitrary