docs: reduce double nesting level in performance.rst

Rendered text was too far to the right.

config: clarify that we only recommend and support single threaded operation

Sad, but true. Especially because we reuse Repository instances between
threads.

middleware: drop gzipper middleware

It added unfortunate buffering, was only enabled for static_files=true, and
isn't available for TurboGears2.

If necessary, it is better to configure the web server to provide it.

config: clarify that we only recommend and support single threaded operation

Sad, but true. Especially because we reuse Repository instances between
threads.

db: get rid of vcs_full_cache - it should always be used

It might make tests less deterministic, but it makes sure that we test what we actually use.

cache: make instance_id = * the default and deprecate it

Auto assigning instance_id's works perfectly fine now when the clean-up issue
has been resolved.

docs: consistent spacing around headings

Start out with 2 empty lines before/after for top level, decrease for deeper
levels.

docs: cleanup of casing, markup and spacing of headings

Mostly stuff found and fixed by Søren, extracted here to separate things.

Other uses of title casing might be debatable, but here it was just a few
documentation headings that clearly were inconsistent with the majority.

docs: improve documentation of beaker session configuration

beaker.session.auto is dropped; it defaults to false and there is no reason to
ever set it true for Kallithea.

beaker.session.cookie_path and secure are dropped; like cookie_domain, they
should automatically be set to the right value.
* * *
beaker.session.cookie_expires MUST have the default value of True to provide the default value of 'browser session lifetime' when not enabling 'remember' in the login box. The cookie life is hardcoded to 365 days when remember is selected.