Refactor message decoding, make keepalive universal

[mraspaud]

This remove the usage of dict for creating messages from strings or deserialisation, deprecates rawstr, introduces the from_string class method.
Also the keepalive options are now systematically applied to all sockets if defined.

[codecov]

Codecov Report

❌ Patch coverage is 91.48936% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.08%. Comparing base (b041234) to head (c5bdb94).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
posttroll/message.py	86.04%	6 Missing ⚠️
posttroll/backends/zmq/subscriber.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #82      +/-   ##
==========================================
- Coverage   88.73%   88.08%   -0.66%     
==========================================
  Files          26       26              
  Lines        2611     2652      +41     
==========================================
+ Hits         2317     2336      +19     
- Misses        294      316      +22     

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