refactor: remove the built-in session manager

[DarthPestilane]

In most cases, we will need our own session manager in project.

[codecov-commenter]

Codecov Report

Merging #32 (6e7f969) into master (f17b5fb) will increase coverage by 0.26%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #32      +/-   ##
==========================================
+ Coverage   96.14%   96.40%   +0.26%     
==========================================
  Files          11       10       -1     
  Lines         363      334      -29     
==========================================
- Hits          349      322      -27     
+ Misses          7        6       -1     
+ Partials        7        6       -1     

Impacted Files	Coverage Δ
server.go	87.34% <100.00%> (+1.13%)	⬆️
session.go	100.00% <100.00%> (ø)

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