Revise use of diode for communicating the target height

[zale144]

Currently, in the retriever, a diode targetSyncHeight which has a buffer size of 1 is used for communicating what the is current highest block size on the DA.

Here we are adding TargetHeight which we use to keep track of what the current highest block is found on both DA and P2P.

This could arguably be simplified, or possibly even merged into a single atomic.Uint64 field, and if needed, using sync.Cond to communicate between goroutines whenever the value is updated.