set default_codec doc attribute

Use shared concurrency for performance / simplicity

Fixes #71

[skip ci] update gemspec summary

Fixes broken link that causes doc build error

Fixes #70

Fix asciidoc problem and do a version bump

elastic/logstash#7993

Fixes #69

bump version (#68)

version bump for some docs fixes

bump major dependency on rabbitmq mixin

Add support for per-message properties

Fixes #66

bump patch level for doc generation