… container ::insert signatures by returning a pointer to the newly created election and also a bool whether the election creation took place.

This allows the call site to retrieve the election that was inserted and determine if it was the initiator of the election creation.
Also updated tests that were taking extra steps to retrieve the newly created election.

… closely match the behavior that's expected.

# Conflicts:
#	nano/core_test/active_transactions.cpp
#	nano/node/blockprocessor.cpp