MINOR: cfgparse: validate defaults proxies separately

MINOR: cfgparse: validate defaults proxies separately

Default proxies validation occurs during post-parsing. The objective is
to report any tcp/http-rules which could not behave as expected.

Previously, this was performed while looping over standard proxies list,
when such proxy is referencing a default instance. This was enough as
only named referenced proxies were kept after parsing. However, this is
not the case anymore in the context of dynamic backends creation at
runtime.

As such, this patch now performs validation on every named defaults
outside of the standard proxies list loop. This should not cause any
behavior difference, as defaults are validated without using the proxy
which relies on it.

Along with this change, PR_FL_READY proxy flag is now removed. Its usage
was only really needed for defaults, to avoid validating a same instance
multiple times. With the validation of defaults in their own loop, it is
now redundant.