Refactor/move tests

[Patrik-Stas]

• Number of integration tests were written on connection handle layer, whereas what these test were really testing was Connection object. So these tests were moved to VCX Connection object layer and adjusted to work with Connection, not with handles
• That also reflected on changes in TestAgent which stored Connection rather than handle
• Unit tests for serialization and deserialization were also moved to work directly with Connection
• Connection::get_invite_details returns Invitation rather than &str
• Moved logic from handle layer, so new function Connection::download_messages was created
• Moved logic from handle layer, so new function Connection::to_string, Connection::from_string were created

[codecov-commenter]

Codecov Report

Merging #289 (e5c9f3b) into master (61d84d6) will increase coverage by 0.02%.
The diff coverage is 36.90%.

@@            Coverage Diff             @@
##           master     #289      +/-   ##
==========================================
+ Coverage   49.79%   49.81%   +0.02%     
==========================================
  Files         153      153              
  Lines       19345    19430      +85     
  Branches     6061     6105      +44     
==========================================
+ Hits         9632     9679      +47     
+ Misses       5016     4999      -17     
- Partials     4697     4752      +55     

Flag	Coverage Δ
integration	22.11% <23.98%> (-0.32%)	⬇️
unittests	45.63% <27.04%> (-0.07%)	⬇️

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Impacted Files	Coverage Δ
libvcx/src/aries/mod.rs	23.07% <ø> (ø)
libvcx/src/lib.rs	0.00% <ø> (ø)
libvcx/src/utils/devsetup.rs	62.66% <ø> (ø)
libvcx/src/utils/devsetup_agent.rs	20.32% <4.08%> (-0.84%)	⬇️
libvcx/src/aries/handlers/connection/connection.rs	43.75% <41.83%> (ø)
libvcx/src/connection.rs	54.32% <61.53%> (+5.04%)	⬆️
...aries/handlers/connection/invitee/state_machine.rs	62.80% <0.00%> (+0.65%)	⬆️

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