Modules: change RedisModuleString API to allow NULL context.
Modules: change RedisModuleString API to allow NULL context.

The burden of having to always create RedisModuleString objects within a
module context was too much, especially now that we have threaded
operations and modules are doing more interesting things. The context in
the string API is currently only used for automatic memory managemnet,
so now the API was modified so that the user can opt-out this feature by
passing a NULL context.

JackieXie168/redis,JackieXie168/redis,JackieXie168/redis,JackieXie168/redis