Simplify the sync SocketBuffer, add type hints

[kristjanvalur]

Pull Request check-list

Please make sure to review and check all of these items:

• Does $ tox pass with this change (including linting)?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?
• Was the change added to CHANGES file?

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Description of change

This change removes two unnecessary state management variables from SocketBuffer,
simplifying the implementation and reducing the chance of programming error. The current
read position is maintained as simply the file_position of the underlying BytesIO buffer. Similarly,
the length of buffered data can be queried simply from the buffer by performing a seek().

Additionally, type hints are added to the SocketBuffer class

[codecov-commenter]

Codecov Report

Base: 92.24% // Head: 92.21% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (518da2f) compared to base (f46d7f3).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2543      +/-   ##
==========================================
- Coverage   92.24%   92.21%   -0.03%     
==========================================
  Files         115      115              
  Lines       29554    29551       -3     
==========================================
- Hits        27262    27251      -11     
- Misses       2292     2300       +8     

Impacted Files	Coverage Δ
redis/connection.py	86.55% <100.00%> (-0.05%)	⬇️
tests/test_asyncio/test_search.py	98.29% <0.00%> (-0.35%)	⬇️
tests/test_cluster.py	96.79% <0.00%> (-0.34%)	⬇️

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[shacharPash]

LGTM 👍