Handle TLS file updates during startup

[jaymode]

This change reworks the loading and monitoring of files that are used
for the construction of SSLContexts so that updates to these files are
not lost if the updates occur during startup. Previously, the
SSLService would parse the settings, build the SSLConfiguration
objects, and construct the SSLContexts prior to the
SSLConfigurationReloader starting to monitor these files for changes.
This allowed for a small window where updates to these files may never
be observed until the node restarted.

To remove the potential miss of a change to these files, the code now
parses the settings and builds SSLConfiguration instances prior to the
construction of the SSLService. The files back the SSLConfiguration
instances are then registered for monitoring and finally the SSLService
is constructed from the previously parse SSLConfiguration instances. As
the SSLService is not constructed when the code starts monitoring the
files for changes, a CompleteableFuture is used to obtain a reference
to the SSLService; this allows for construction of the SSLService to
complete and ensures that we do not miss any file updates during the
construction of the SSLService.

While working on this change, the SSLConfigurationReloader was also
refactored to reflect how it is currently used. When the
SSLConfigurationReloader was originally written the files that it
monitored could change during runtime. This is no longer the case as
we stopped the monitoring of files that back dynamic SSLContext
instances. In order to support the ability for items to change during
runtime, the class made use of concurrent data structures. The use of
these concurrent datastructures has been removed.

Closes #54867
Backport of #54999

[elasticmachine]

Pinging @elastic/es-security (:Security/Network)