Bug fixes

Fixes a bug where the calico/cni container would ignore termination signals. cni-plugin #487 (@ketkulka)
Closes a number of race conditions and failure scenarios in IPAM block allocation and releasing. libcalico-go #819 (@caseydavenport)
Improves log output around IPAM block allocation and releasing. libcalico-go #819 (@caseydavenport)
Fixes a bug where IPs could be assigned from disabled IP pools. libcalico-go #819 (@ozdanborne)
Fixes a rare bug where a node could, in some circumstances, advertise /26 blocks that it didn't own calico #1751 (@caseydavenport)

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