[electrs] never finishes the indexing

I tried to run Electrs light mode indexing on two different machines, one with macOS, another with Debian. While it initially starts indexing after few hours it doesn't seem to do anything. IO and CPU usage are about zero. But the programs still cannot be stopped (only with `kill -9`) and doesn't open port for API. I tried multiple times with each install, restarting with an existing db, or from scratch.

It seems to be a problem related to `--lightmode`, because without this option I didn't notice this issue, but I _always_ see it with the light mode. It always stuck at the end with the light mode and never continues, I tried to wait for up to 7 days without luck.

The last messages in the debug output are like (when I restart it with existing db):
```
...
TRACE - skipping block 00000000000000000000b26e70fb11b6d57f6d2285f31b32a86347ef6d413d9e
TRACE - skipping block 0000000000000000001002729e15e01a540b6fe27ef44aa6a8663a0ee4ce761e
TRACE - skipping block 00000000000000000009557f23813c41f80db2386a172ab5e7f1df8a5c710f87
TRACE - fetched 0 blocks
DEBUG - writing 0 rows to RocksDB { path: "/electrs/mainnet/newindex/history" }, flush=Disable
TRACE - skipping block 0000000000000000000a1d16a854aaeebb8ea7513fd4c08cd728b0afa50d572d
TRACE - fetched 61 blocks
DEBUG - writing 859107 rows to RocksDB { path: "/electrs/mainnet/newindex/history" }, flush=Disable
DEBUG - flushing to disk
```

As I mentioned, I also tried to run without `--lightmode` and had no problem (except extending open files limit, but that's understandable)