[RFC v3] fiber: Increase default stack size

[Vladimir Davydov]

On Tue, Feb 26, 2019 at 01:26:56PM +0300, Vladimir Davydov wrote:
>  - 1 MB for max stack size seems to be a bit of an overkill for now.
>    The default value should be set to 256 KB, but we do need a
>    configuration option for it. Let's add it to the fiber Lua module.
>    May be done in a separate patch, but should be submitted together in
>    the same patch set.

Almost forgot. We should also print a warning to the log if some fiber
has gone too close to the stack limit (say half of the max allocation)
to indicate that the max stack size should be increased.