pgAdmin 4 commit: Make the session thread safe.

Make the session thread safe.

As sessions in pgAdmin4 are filesystem based session, they need locking
for avoiding the access from multiple threads, specially running as an
WSGI application.

Fixes #3547

Branch
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master

Details
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https://git.postgresql.org/gitweb?p=pgadmin4.git;a=commitdiff;h=013ad7446fdfa7367b4d580d9e9d53dfd98d0316
Author: Harshal Dhumal 

Modified Files
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docs/en_US/release_notes_3_3.rst |  1 +
web/pgadmin/utils/session.py     | 76 +++++++++++++++++++++++-----------------
2 files changed, 44 insertions(+), 33 deletions(-)