Use TS::Cache instead plain Hash in TimeZone.

Plain ruby Hashes are not thread safe.
Unify AS::TZ's lazy init maps.

There's no point in having 2 almost identical (@lazy_zones_map and @zones_map) lazy initialized TZ instance caches.
Remove AS::TZ.lookup(name).

The method doesn't really make much sense (find_tzinfo will succeed for any imput provided).
Move Rack::Cache after AD::Static in the stack
Fix segmentation fault in Ruby 2.0.0-p353.

In Ruby 2.0.0-p353 there was a
[commit](https://github.com/ruby/ruby/commit/66915c507777c5e3a978fa73de25db763efd9206)
that switched case matching from actual sending `===` method to magic lookup,
that does not see it in `method_missing`. It's hard to predict how exactly and
when exactly this bug will be solved so here I suggest a solution of defining
it in Duration directly.

In Ruby 2.0.0-p353 without the added fix added test crashes to segmentation
fault.
Typo fixes [ci skip]
Merge pull request #13111 from akshay-vishnoi/typo

Typo fixes [ci skip]
Improve AR changelog [ci skip]
Merge pull request #13055 from dmitriy-kiriyenko/fix-segmentation-fault-it-ruby-2.0.0-p353

Fix segmentation fault in Ruby 2.0.0-p353.
Update render with a spacer_template [ci skip]
Merge pull request #13119 from sbagdat/patch-1

Update render with a spacer_template [ci skip]
Removed redundant field name in the guide 

Fixes: #13108
[ci skip]
Merge pull request #13121 from pftg/patch-1

Removed redundant field name in the guide
Honor to RAILS_GROUPS env variable while loading gems with Bundler
config.assets.enabled isn't used anymore to disable Rails asset pipeline
Merge pull request #11796 from thedarkone/time-zone-thread-safety

AS::TimeZone's cache thread safety
Use genderless pronouns in API docs
EscapedString is also private API [ci skip]
Merge pull request #13123 from chancancode/json_encoder_nodoc_fix

JSON encoder internals showed up in docs [ci skip]
Use .railsrc while creating new plugin if available

- Fixes #10700