eth/filters: annotate logs in a sub with block end markers

[karalabe]

EDIT: Ugh, so this doesn't actually work, because the *types.Log embedding redefines JSON marshalling, which overrides it even for the wrapper. Ugh

Fixes #24787

When subscribing to logs, the user will get a never ending stream of them. It can be annoying to figure out whether the last log of a block arrived, or if there's some more in the pipeline. This PR adds a block end marker into logs.

[holiman]

From @fjl

We can add field Last in types.Log
With omitempty)
Clients will also need it, and the type used in subscription is types.Log

[karalabe]

So, to make this work we should:

• Add the Last to types.Log
• Set it during loading from the db
• Set it whenever we refilter

[fjl]

Closing because #26109 is more up-to-date.

[holiman]

Closing because #26109 is more up-to-date.