Add a maximum batch size for regular sync imports

[carver]

Without this maximum batch size, when the first batch takes a long time
to execute, the second batch will be enormous. Perhaps most importantly,
the back-pressure set by _db_buffer_capacity is never triggered if the
batch is unlimited size. So the header downloader races ahead unimpeded.

I think this was something that @veox noticed.

Note that beam sync uses the regular syncer updated in this PR, which is why it was originally bundled in #641 .

To-Do

• Add entry to the release notes PR

Cute Animal Picture

[veox]

I think this was something that @veox noticed.

Yes, but it seems I never filed an issue.

[cburgdorf]

Oh! This is an interesting one! While working on the isolated process sync I had seen cases where the header sync just races too far ahead and I couldn't figure out how to deal with it (and the problem wasn't that apparent anymore at some point).

LGTM 👍