introduce Sync() method on blockstores

[raulk]

Goal

To reduce the amount of fsyncing by opening the blockstores (lmdb and badger) in "no sync" mode, and introducing explicit fsyncs only when we're effectively committing new state (i.e. writing a new head).

See discussion in #5050.

Design

1. Change the blockstore options to enable NoSync mode.
2. Expand the lotus/lib/blockstore.Blockstore interface to introduce a Sync() method.
3. Call Sync() from ChainStore#takeHeaviestTipSet, which is when we commit a new head and thus want all previously unflushed chain and state objects to become durable.

[jennijuju]

We should consider revert #6817 if this is implemented