Since external diffs are likely to be a bit slower than in-database
ones, add a mode that makes diffs external after they've been obsoleted
by events. This should strike a balance between performance and disk
space.

A background cron drives the majority of migrations, since diffs become
outdated through user actions.

In this commit, some methods that aren't being used
are removed from `Gitlab::Shell`. They are the ff:
- `#remove_keys_not_found_in_db`
- `#batch_read_key_ids`
- `#list_key_ids`

The corresponding methods in `Gitlab::Keys` have been
removed as well.

We've previously exposed ca_file and ssl_version but there are many
possible options that can be used inside tls_options. Instead of
exposing individual ones, simply expose the entire hash so it can
be passed in and we won't have to add things in the future.

Signed-off-by: Rémy Coutable <remy@rymai.me>

Adds gitlab.impersonation_enabled config option defaulting to true to
keep the current default behaviour.

Only the act of impersonation is modified, impersonation token
management is not affected.