minor optimisations

[dapper91]

• initialize dispatcher middleware stack in constructor.
• generic context type added.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 75.64103% with 19 lines in your changes missing coverage. Please review.

Project coverage is 79.08%. Comparing base (4bd7a02) to head (3b11fd2).

Files with missing lines	Patch %	Lines
pjrpc/server/integration/starlette.py	0.00%	8 Missing ⚠️
pjrpc/server/integration/aio_pika.py	0.00%	3 Missing ⚠️
pjrpc/server/integration/django/sites.py	0.00%	3 Missing ⚠️
pjrpc/server/integration/kombu.py	0.00%	3 Missing ⚠️
pjrpc/server/integration/aiohttp.py	85.71%	1 Missing ⚠️
pjrpc/server/integration/flask.py	85.71%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #119      +/-   ##
==========================================
- Coverage   79.15%   79.08%   -0.08%     
==========================================
  Files          42       42              
  Lines        2975     2983       +8     
==========================================
+ Hits         2355     2359       +4     
- Misses        620      624       +4     

Flag	Coverage Δ
unittests	79.08% <75.64%> (-0.08%)	⬇️

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