BUG/MINOR: hlua: use CertCache.set() from various hlua contexts

BUG/MINOR: hlua: use CertCache.set() from various hlua contexts

Using CertCache.set() from init context wasn't explicitly supported and
caused the process to crash:

crash.lua:
  core.register_init(function()
    CertCache.set{filename="reg-tests/ssl/set_cafile_client.pem", ocsp=""}
  end)

crash.conf:
  global
    lua-load crash.lua
  listen front
    bind localhost:9090 ssl crt reg-tests/ssl/set_cafile_client.pem ca-file reg-tests/ssl/set_cafile_interCA1.crt verify none

./haproxy -f crash.conf
[NOTICE]   (267993) : haproxy version is 3.0-dev2-640ff6-910
[NOTICE]   (267993) : path to executable is ./haproxy
[WARNING]  (267993) : config : missing timeouts for proxy 'front'.
   | While not properly invalid, you will certainly encounter various problems
   | with such a configuration. To fix this, please ensure that all following
   | timeouts are set to a non-zero value: 'client', 'connect', 'server'.
[1]    267993 segmentation fault (core dumped)  ./haproxy -f crash.conf

This is because in hlua_ckch_set/hlua_ckch_commit_yield, we always
consider that we're being called from a yield-capable runtime context.
As such, hlua_gethlua() is never checked for NULL and we systematically
try to wake hlua->task and yield every 10 instances.

In fact, if we're called from the body or init context (that is, during
haproxy startup), hlua_gethlua() will return NULL, and in this case we
shouldn't care about yielding because it is ok to commit all instances
at once since haproxy is still starting up.

Also, when calling CertCache.set() from a non-yield capable runtime
context (such as hlua fetch context), we kept doing as if the yield
succeeded, resulting in unexpected function termination (operation
would be aborted and the CertCache lock wouldn't be released). Instead,
now we explicitly state in the doc that CertCache.set() cannot be used
from a non-yield capable runtime context, and we raise a runtime error
if it is used that way.

These bugs were discovered by reading the code when trying to address
Svace report documented by @Bbulatov GH #2586.

It should be backported up to 2.6 with 30fcca18 ("MINOR: ssl/lua:
CertCache.set() allows to update an SSL certificate file")