Bump default VM atom table limit to 5M

[michaelklishin]

See #895 for background and reasoning.

Fixes #895.

[kjnilsson]

Ok I’ve started the service on windows with that setting and verified it using: rabbitmqctl eval 'string:tokens(binary_to_list(erlang:system_info(info)),"\n").'

In the output I found this:

32665\ndepth: 10\n=i", "dex_table:atom_tab\nsize: 32768\nlimit: 5000000\ne"

I think I got the escaping of the token delimiter on windows a bit wrong but it does show the atom_tab limit to be of the correct value.