fix agents with multiple topics

[stevanmilic]

Description

A bug was introduced in fda7e52
where an asyncio task was using a topic name as its identifier. The
change isn't compatible with the Topic implementation where ValueError
is thrown for getting topic name when multiple names are set.

[codecov-io]

Codecov Report

Merging #116 (4c3303e) into master (5b68580) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #116      +/-   ##
==========================================
- Coverage   93.75%   93.74%   -0.01%     
==========================================
  Files         100      100              
  Lines       10627    10619       -8     
  Branches     1196     1194       -2     
==========================================
- Hits         9963     9955       -8     
  Misses        611      611              
  Partials       53       53              

Impacted Files	Coverage Δ
faust/agents/agent.py	98.06% <100.00%> (-0.05%)	⬇️

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