2.3.0

• Add optional how parameter to thread list init signature, @AndreasDahl
• Upgrade min required async version to v0.16.0, @AndreasDahl
• Replace usage of deprecated Core.Time with Core.Time_float, @AndreasDahl
• Allow creation of internal exchanges (only useful for use with rabbitmq). Thanks hongchangwu
• Update amqp spec
• Remove dependency on ocplib-endian
• Bump minimum ocaml version to 4.14