core: Retry bonding attempt until the iterator reaches the end

core: Retry bonding attempt until the iterator reaches the end

This patch splits the bonding process in an interative process
consisting of one or more "bonding attempts". The user/agent starts a
new "bonding" that may involve several rounds of "bonding attempts" with
the device before it gets back to the user/agent.

Some functions were split in two parts to reflect this change. When a
bonding attempt fails with an authentication error and a pin code was
provided, a new bonding attempt is initiated (after a short delay) to
retry with the next function (or next call to the same function) in the
pincode callback list. If no pin code was provided to the device, no
retry is attempted and the authentication error is returned to the
client. This effectively allows a plugin try different pincodes for the
same device during the same bonding.