Older versions might not have this function.

Those calls may be subject to changes in the future, so the user should
acknowledge it is using non stable API.

As suggested by @itamarhaber.

It was noted by @dvirsky that it is not possible to use string functions
when writing the AOF file. This sometimes is critical since the command
rewriting may need to be built in the context of the AOF callback, and
without access to the context, and the limited types that the AOF
production functions will accept, this can be an issue.

Moreover there are other needs that we can't anticipate regarding the
ability to use Redis Modules APIs using the context in order to build
representations to emit AOF / RDB.

Because of this a new API was added that allows the user to get a
temporary context from the IO context. The context is auto released
if obtained when the RDB / AOF callback returns.

Calling multiple time the function to get the context, always returns
the same one, since it is invalid to have more than a single context.

When double precision is not needed, to take 2x space in the
serialization is not good.

RedisModule_StringRetain() allows, when automatic memory management is
on, to keep string objects living after the callback returns. Can also
be used in order to use Redis reference counting of objects inside
modules.

The reason why this is useful is that sometimes when implementing new
data types we want to reference RedisModuleString objects inside the
module private data structures, so those string objects must be valid
after the callback returns even if not referenced inside the Redis key
space.

This commit changes what provided by PR #3315 (merged) in order to
let the user specify the log level as a string.

The define could be also used, but when this happens, they must be
decoupled from the defines in the Redis core, like in the other part of
the Redis modules implementations, so that a switch statement (or a
function) remaps between the two, otherwise we are no longer free to
change the internal Redis defines.

module.