debugger_v2: start multiplexer lazily

[wchargin]

Summary:
The DebuggerV2 plugin reads data outside of the usual summary-based
workflows, and has its own loading infrastructure for doing so. Until
this patch, the plugin would start recursively scanning the user’s log
directory at plugin initialization time—even before the server launches.
This can take a lot of resources and introduce a lot of contention,
especially on remote file systems.

This patch defers all such logic until users of the frontend explicitly
activate the plugin. Startup time improves greatly (see below).

Test Plan:
When pointing at gs://tensorboard-bench-logs, the time between process
start and the “server ready” line drops from 9.5 seconds to 1.7 seconds,
as measured by:

bazel-bin/tensorboard/tensorboard --logdir gs://tensorboard-bench-logs 2>&1 \
    | ts -s '[%.s]'

After generating test data as described in the tutorial, the plugin is
indeed inactive by default, and does indeed still appear to work if
manually activated.

wchargin-branch: dbgv2-lazy-load