Add support for 'nocache' flag to map directive

[Eran Kornblau]

> 
> From: nginx-devel [mailto:nginx-devel-bounces at nginx.org] On Behalf Of Sorin Manole
> Sent: Tuesday, November 29, 2016 7:19 AM
> To: nginx-devel at nginx.org
> Subject: Re: Add support for 'nocache' flag to map directive
> 
> Would it make sense to add nocacheable if input_var or one of the variables in the value options is nocacheable?
> 
Yes, that makes perfect sense to me, but that will be harder to implement, for example, 
when the map block is initialized, I'm not sure whether all the variables it references have
already been added. If they were not, it will not know they are 'nocacheable', and it will require
some post processing step (maybe in postconfiguration) to go over the tree of variable references
and update nocacheable (there could be a map block that uses a variable created in another map block etc.)

So, while not ideal, I think adding a flag that the user will have to manually set is an acceptable compromise. 
If the forum prefers the more automated solution, I'm willing to try to tackle that as well, but will need some 
high level guidance on how you'd like that implemented.
Please let me know what you think

Thank you,

Eran