Replace json std package with github.com/json-iterator/go

[rustatian]

1. Refactor amqp/conn, move redial from config to conn.go
2. Remove dialing function, as not concurrent safe, refactor it
3. Simplify conn.notify function
4. Add a simple threshold limiter to BS/conn.go. Limit located in the constants.go file in beanstalk package

[wolfy-j]

bors r+

[bors[bot]]

Build failed

• Build (PHP 7.3, Go 1.13, OS ubuntu-latest)

[rustatian]

bors r+

[bors[bot]]

Build succeeded

• Build (PHP 7.2, Go 1.13, OS ubuntu-latest)
• Build (PHP 7.2, Go 1.14, OS ubuntu-latest)
• Build (PHP 7.3, Go 1.13, OS ubuntu-latest)
• Build (PHP 7.3, Go 1.14, OS ubuntu-latest)
• Build (PHP 7.4, Go 1.13, OS ubuntu-latest)
• Build (PHP 7.4, Go 1.14, OS ubuntu-latest)
• runner / golangci-lint

[rustatian]

resolves #38