BTC height indexing / Undelegate expired delegations automatically

[maurolacy]

Currently the BTC height is only used during slashing, so there's no need to store it at each Consumer height (BTC height indexing disabled in #64).

Re-add BTC height indexing when needed. By example, to detect expired timelock delegations independently of Babylon, and to automatically undelegate them.

Note: This feature (auto undelegations) could be put into a full feature flag, as the alternative impl could be, to simply receive those expired delegation undelegations from Babylon. In that case, BTC height indexing wouldn't in principle be needed.