eth/downloader: increase downloader block body allowance (#23074)

This change increases the cache size from 64 to 256 Mb for block bodies.
Benchmarks have shown this to be one bottleneck when trying to achieve
higher download speeds.

The commit also includes a minor optimization for header inserts in package
core: previously, the presence of headers in the database was checked for
every header before writing it. With the change, if one header fails the
presence check, all subsequent headers are also assumed to be missing.
This is an improvement because in practice, the headers are almost always
missing during sync.