Avoid concurrency issues with load_metadata function

[jgsogo]

Changelog: omit
Docs: omit

Use the locking mechanism to read the metadata file, not only with the contextmanager when you are going to modify it.

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I'm trying to figure out which could be the worst scenario that leads to the assert failure:

assert node.prev, "PREV for %s is None: %s" % (str(pref), metadata.dumps())

[jgsogo]

Bitten by harlowja/fasteners#26 and the layout we are using to store the lockfiles and determine if a package exists. Not sure if it will require too many changes 😞

[jgsogo]

Alternatives:

• tell fasteners to store the files somewhere else
• force removal of the metadata.json.lock file if it is the only one in the folder here (we will face a concurrency challenge):

  conan/conans/util/locks.py

  Line 109 in 8e8ebce

  try:

[memsharded]

Obsolete, closing