* fix obtaining deposits after connection loss
When an error occurs during Eth1 deposits import, the already imported
blocks are kept while the connection to the EL is re-established.
However, the corresponding merkleizer is not persisted, leading to any
future deposits no longer being properly imported. This is quite common
when syncing a fresh Nimbus instance against an already-synced Geth EL.
Fixed by persisting the head merkleizer together with the blocks.
4ef621f926