rbssh has historically executed within a Review Board environment in

order to take advantage of extension support, necessary in order for the

flexible SSH key storage backends to take effect. Originally, startup

time was fairly reasonable, but over time this became more expensive,

and on large deployments (particularly those using many extensions, or

expensive-to-load extensions) it could be a bottleneck.
This change reworks execution by running in a minimal Django

environment. It now uses a custom settings.py, which loads only the

bare-minimum Django apps needed to manage and implement SSH key storage

backends.
This means that extensions are no longer loaded. In order for an

extension-supplied storage backend to still work, the supplier of that

backend must set the siteconfig-provided ssh_storage_backend to the

path of the backend class. This supercedes the now-legacy

RBSSH_STORAGE_BACKEND key in settings.py. It also means that

extensions must be updated to set the new key in order to continue

working. Unlike the old process-local setting, the new setting

persists in the database, allowing rbssh to look it up without loading

any extensions.
Backend loading will try the new setting and the old setting, which will

help when performing SSH-backed requests from within the Review Board

process, or in rbssh if settings_local.py defines

RBSSH_STORAGE_BACKEND.
SSH communication within rbssh has also received some small speed

tweaks. We no longer attempt to log anything to debug if the

$DEBUG_RBSSH environment variable isn't set. We previously would send

to the logger, which would go through the internal logging processes and

then get ignored. We now just don't do anything.
NOTE: This is a breaking change:


Extension developers providing custom SSH key storage backends will

   need to ensure their backends run within a bare-minimum environment.

   They can rely on the Django Site and Djblets SiteConfiguration

   models, but nothing else. If this impacts anyone, they will need to

   talk to us about a future-proof solution, but this change is not

   currently attempting to over-engineer this, since we're only aware of

   the custom backend provided by Power Pack.


Power Pack users will need to update to Power Pack 3.0.3 (still

   in-development at the time of this change) in order to continue using

   DB-backend SSH keys, or will need to set the following in

settings_local.py:



RBSSH_STORAGE_BACKEND = 'rbpowerpack.sshdb.storage.DBSSHStorage'

Unit tests pass.
Manually invoked rbssh, running speed tests and comparing the old

execution time to the new one. Saw a drastic speed reduction, particularly

with extensions enabled.
Tested usage from within Review Board.
Tested with the default local keys backend and the Power Pack SSHDB backend

(using an updated Power Pack with support for the new setting), verifying

that the INSTALLED_APPS changes didn't break anything.
Tested these changes on both Review Board 3.0 and 4.0.