Start deposit tree sync only after beacon node is synced

[pawanjay176]

Description

Before the merge, since the the EL and CL systems were separate, we could do an eth1 deposit tree sync concurrently with the beacon node sync assuming that the EL is already synced. This assumption is not true after the merge as the EL and CL together make up an ethereum node and come to sync together.

Hence, all the requests we make to sync the deposit cache before the main sync is completed will return errors. So we should only make requests to sync the deposit tree after main sync is completed.

This might be related to @ethDreamer 's work on syncing the deposit tree directly over p2p.