TS-2212 Implement <fsiz> tag for HTTP

The current docs explains it as

``fsiz``
    The size of the file (*n* bytes) as seen by the origin server.

However, there's nothing that implements this. This code will
try its best to find a body size, based on origin response
headers such as Content-Range and bytes read.

It's somewhat unfortunate that at the time of logging, the CacheVC
is no longer evailable, and all headers are already modified such
that they reflect client's e.g. Range: bytes=  header.

This patch implements the logic around using the Range-response
headers when available, and the internal (full) byte counters
when not.

Finally, this also generalizes the "atoi" functionality in lib/ts
(ParseRules) such that the prototypes with and without a string
length are available for all int types. This was needed in order
to do an efficient and safe atoi on the not-null terminated string
to a 64-bit int.
TS-2266 Add a "make rat" Makefile target, to create a RAT report
Remove long dead config options that were always false
DOC: cache document tweaks.
Thundering herd section-- source code block doesn't eval links. Trying line blocks instead
TS-2268 Add support for opening protocol traffic sockets through the
traffic_manager.
DOC: Added HostDB architecture proposal. Added proposal section to architecture area.
DOC: Tiered storage proposal update. Removed HostDB proposal reference.
Thundering herd section-- background fill description error
Fix auto-numbering formatting
TS-2270 ESI Plugin can have infinite loop on gunzip

Reviewed: leif

Note: I'm not an expert on gzip, but these changes seems
inline with how we deal with this in the gzip plugin.
TS-2269 regex_remap plugin does not deal with empty path's properly.

This is a problem when the request URL path is just '/', which inside
of ATS is an emptry string.

Review: leif
Added TS-2269.
TS-2254 On ARM arch, ink_atomic_increment returns wrong value

It should return the old value, before the increment. This is
consistent with how the other ink_atomic_increment implementation
works.

Review: leif
Added TS-2254.
TS-2189 Document proxy.config.http.cache.cluster_cache_local.

This is confusing for sure, in that it's actually an alternative to
controlling local caching via records.config. The intent, as far as I can
tell, is that you'd use this as an overridable configuration.
TS-2189 Document cluster-cache-local in cache.config
TS-2197

use HttpSM::main_handler to handle the client request stuff ranther
than directly call the HttpSM::state_read_client_request.

reenable the client_vc even if the ua_buffer_reader is not empty, or
else we may not perceive client_vc`s aborted.
DOC: Large update of cache architure.
doc: update cluster howto to for connections