[Enhancement] Improve BackupReady conditions

[shreyas-s-rao]

Feature (What you would like to be added):
Enhance BackupReady conditions to provide more meaningful messages and reasons for setting different BackupReady conditions.

Motivation (Why is this needed?):
Currently not all combinations of full and delta snapshot lease renewal times are handled by custodian controller.

Approach/Hint to the implement solution (optional):
Add additional field state to EtcdMember.status.snapshots.lastFull and EtcdMember.status.snapshots.lastDelta, in order to allow the etcd member to correctly publish snapshot state information, which druid will use to accurately populate the BackupReady condition, as described in #619 (comment)

[shreyas-s-rao]

/assign

[plkokanov]

We could also improve the BackupReady condition by providing additional information (either via logs or the conditions messages field) when it's status is Unknwon ref: #732