Add additional flags to dialyzer config

[starbelly]

This commit adds :unmatched_returns and :race_conditions to dialzyer config.

See https://www.erlang.org/doc/man/dialyzer.html for details

• changed lib/mllp/sender.ex to cancel timer asynchronously

[starbelly]

Note : I need to squash this before merging as the original commit message needs to be altered per switching from async: true to info: false