Check security properties in the main transaction

[nickva]

Previously we checked security properties in a separate transaction, after
opening the db or fetching it from the cache. To avoid running an extra
transaction move the check inside the main transaction right after the metadata
check runs. That ensure it will be consistent and it won't be accidentally
missed as all operations run the ensure_current metadata check.

Also remove the special get_config/2 function in fabric2_fdb for getting
revs limit and security properties and just read them directly from the db map.

Test with:

make && make eunit apps=fabric,couch_views,couch_jobs

make && make elixir tests=test/elixir/test/basics_test.exs,test/elixir/test/replication_test.exs,test/elixir/test/map_test.exs,test/elixir/test/all_docs_test.exs,test/elixir/test/bulk_docs_test.exs

[nickva]

Quick benchmark with this PR:

prototype/fdb-layer (FDB master):

time ./add_docs.py
> db url: http://adm:pass@127.0.0.1:15984
> db:info {u'update_seq': u'00000002313b676000000000', u'disk_size': 0, u'sizes': {u'active': 0, u'external': 2, u'file': 0}, u'purge_seq': 0, u'doc_count': 10000, u'compact_running': False, u'cluster': {u'q': 0, u'r': 0, u'w': 0, u'n': 0}, u'db_name': u'db', u'disk_format_version': 0, u'instance_start_time': u'0', u'other': {u'data_size': 2}, u'data_size': 0, u'doc_del_count': 0}

real	0m55.825s
user	0m10.583s
sys	0m3.986s

This PR:

time ./add_docs.py
> db url: http://adm:pass@127.0.0.1:15984
> db:info {u'update_seq': u'00000002313b676000000000', u'disk_size': 0, u'sizes': {u'active': 0, u'external': 2, u'file': 0}, u'purge_seq': 0, u'doc_count': 10000, u'compact_running': False, u'cluster': {u'q': 0, u'r': 0, u'w': 0, u'n': 0}, u'db_name': u'db', u'disk_format_version': 0, u'instance_start_time': u'0', u'other': {u'data_size': 2}, u'data_size': 0, u'doc_del_count': 0}

real	0m34.587s
user	0m10.486s
sys	0m4.118s

add_docs.py: https://gist.github.com/nickva/5d88c389d45ab538950a5e0f34b7d2ae

First ran script once to create and populate db then run again to read the documents

With the chttpd_auth_request security check back in got:

real	0m44.132s
user	0m11.676s
sys	0m4.437s

[davisp]

Looks good overall. Few questions though.

[davisp]

One minor nit, but +1 once you tweak the map update operations.