Isolate Bokeh state for concurrent plot output

[DirkEmmett]

Concurrent plots share Bokeh's global output state, so one request can overwrite another request's HTML file or leave it missing.

Use per-call state and documents for plots and heatmaps, preserving notebook callbacks and delayed BokehJS loading.

Fixes #125.

Validation: 85 tests passed, 1 skipped on both Bokeh 3.10.1 and 3.1.1; the 3.10.1 browser smoke passed. Existing Mypy errors and the 3.1.1 browser-smoke failure also reproduce on the base revision.

[arhancanli]

Read the diff. I did not run the tests; I only checked what I could outside the suite.

The notebook branch of _show re-implements part of Bokeh's show_doc() with four non-public imports: bokeh.core.json_encoder.serialize_json, bokeh.core.templates.DOC_NB_JS, bokeh.embed.elements.div_for_render_item and bokeh.embed.util.standalone_docs_json_and_render_items. I downloaded the wheels and all four are present in Bokeh 3.1.1, 3.3.4 and 3.4.3 (the only versions I could fetch with Python 3.9), plus 3.10.1 per the PR's own run. That covers the two ends of the range setup.py allows (bokeh >= 3.0.0, != 3.0.*, != 3.2.*) but not 3.5 to 3.9, and the PR's validation names only 3.1.1 and 3.10.1. Since these are internals that Bokeh may move, a CI cell for one version in the middle would show early if one of them goes, and a comment in _show pointing at show_doc as the thing being mirrored would help whoever has to repair it.

The file path (save(fig, state=state) on a fresh State() per call) is the part that fixes #125, and test_concurrent_plot_files exercises it with a barrier inside gridplot, which forces the two calls to overlap. That is a good way to test it.