HaikuDepot: Url -> Identifier

This change will rename the confusing "url" within
HaikuDepot to be "identifier" in line with
corresponding changes in pkg kit and HDS. Also at
the same time support is introduced for HDS
repos' meta-data to artificially match against
multiple repos; as requested for the future R1B3
release process. Some tidy-ups and extensions have
been made to the JSON schema-to-model and the
schema-to-parser scripts.

Change-Id: I402e7d610986039f58d72028bda7de977e9115e2
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2986
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>

HaikuDepot : More Backend Communications Improvements

* Further improves the logging and provides some
basic performance numbers.
* Moves the bulk-load logic out of the data-model
class.
* Introduces a state-machine for the bulk-load
process so that it will be more easily able to be
shifted to non-blocking IO when the HTTP libraries
can do that.
* Implements concurrent loading of the bulk-data to
hopefully improve lead time for icons and meta-data.
* Loads data to a temporary file and then moves to the
final location in order to avoid partially written
data in the cache.
* Handles situations where no network is available;
prevents attempt to access the network.
* Allows bulk-load processes to be cancelled when the
application quits.
* Introduces command-line arguments to help simulate
scenarios to help with testing performance and
network absence.
* Implements ordered insert and binary search in the
'List' class + basic unit test.

HaikuDepot: Change communication mechanism with server for repos

Previously the desktop application would make a number of JSON-RPC calls
over HTTP to get the repositories. Now it will make a single call to get
the repositories and cache the result. This uses standard HTTP cache
signalling techniques and allows the server-side the ability to cache
the generated data as well. Note that the model classes and parse-
related classes are generated and may not be code-style compliant. They
are generated from JSON schema files in the server-side project.
Information about this as well as the python files used to generate the
C++ classes and headers are included in the server-side project.