Rename "Drop*" to "Retry*"

[arnecls]

The term Drop lead to a lot of confusion.
Most people thing of "drop" as an destructive operation (like in mysql, ip, etc.). As of this drop was mistaken as a discard in the past quite often.
The "Drop" mechanism was originally thought of as kind of an exception handling mechanism: If you cannot deliver a message, drop it, so somebody else might catch it. So basically it implements a "retry by routing" operation.

• Find all occurrences of "Drop*" and replace them by "Retry*"
• Make it clear that this operation is a "retry in case of error" in the documentation