Use smaller batches when inserting to postgres.

[winder]

For large blocks, inserting transactions and updating accounts can be sped up by sending multiple smaller batches of updates instead of one large batch.

Modifying the batches can be done for all updates, but probably would have the biggest impact for these tables:

• transaction inserts
• account upserts

A proof of concept was made here: winder@1d5edb2

Here are some rough notes from tests done with the above branch repeatedly inserting the same full block. Unsafe refers to the serialization level.

Note	Vacuumed	AddBlock	Txn Participation	AddTransactions	Overall
Baseline		401.9ms	1.9s	29s
Baseline 2	X	390ms	1s	26.4s
Rollback instead of commit	X	82ms	984ms	3s
Unsafe Serialization (txns)	X	247ms	1.3s	23.7s
Unsafe Serialization (all)
Unsafe + Txn batches (10k)		193.6ms	1.5s	9s
Unsafe + Txn batches (1k)	X	214ms	7.1s	7.7s	8.7s
Unsafe + Txn batches (2k)	X	223ms	4.8s	6.6s	7.6s
Unsafe + Txn batches (2k)	X (full)	233ms	2.8s	7.1s	8.3s