Expose H5PL low-level API

[takluyver]

This is a continuation of #1166. I've updated it to use the @insubprocess decorator @scopatz added, fixed the tests running in a conda environment (conda-forge configures a non-default plugin directory when building HDF5), and added some docs for the new functions.

Remaining questions:

1. Currently the path APIs accept and return bytes. Although this is technically correct for Unix paths, it's more common in Python to work with paths as strings. We're a bit inconsistent elsewhere - the low-level API for an external link returns bytes, but getting the source of a virtual dataset returns str. I'm leaning towards staying with bytes in the low-level API.
2. Do we want to do any locking? On H5PL: Plugin Interface #1166, @tacaswell said they shouldn't need the phil lock, but there might be reasons to have a separate lock for the plugin search path:

We may also want to lock them between eachother (as I can imagine an append and a remove colliding and ending up with a "gap" in the hdf5 datastrucutres....).

I'm tentatively marking this for 2.10, but I don't mind if we bump it to a later release.

[scopatz]

This looks good to me! Thanks @takluyver!

there might be reasons to have a separate lock for the plugin search path

Does HDF5 have a lock of its own around this?

[takluyver]

I can't see locking in a quick look at the HDF5 source code, though it's possible that I'm overlooking it in a macro or something. Maybe we should just document for now that it's very likely not thread-safe.

[scopatz]

Maybe we should just document for now that it's very likely not thread-safe.

Seems reasonable for now

[codecov]

Codecov Report

Merging #1256 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1256   +/-   ##
=======================================
  Coverage   83.82%   83.82%           
=======================================
  Files          18       18           
  Lines        2170     2170           
=======================================
  Hits         1819     1819           
  Misses        351      351

Impacted Files	Coverage Δ
h5py/__init__.py	59.64% <100%> (ø)	⬆️

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[scopatz]

Thanks @takluyver!