loading data in tikv gets stuck sometimes: batch is pushed when > size

[matthewdarwin]

This is a followup to #161

Loading data into tikv gets stuck sometimes, and then may eventually work. The batches are sometimes too big.

The batch is pushed when > size, but we should probably detect if it would be > size, flush the current batch and then start a new one with the item.

[matthewdarwin]

You get messages like this:

2020-10-01T21:36:01.277Z (statedb.js.live) source shutting down (blockstream/source.go:154){"error": "file source failed: process block failed: flushing if full: flushing batch set: unable to add table '\\x00' key \"\\xb0\\x00U1\\xb3\\\"\\xb4\\xa4B\\x10K\\xb1\\x9b\\xb8\\xfa˩0U1\\xb3\\\"\\xb4\\xa4B\\x10\\x00\\x00\\x00\\x00\\b\\xa1\\xf2\\xd1\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\" to tx: message:\"raft entry is too large, region 101282410, entry size 8553531\" raft_entry_too_large:<region_id:101282410 entry_size:8553531 > "}

[sduchesneau]

fixed by @maoueh 204a47e