Optimize rebalance by reducing token transfers

[saucepoint]

for future iterations --

when Doppler is moving liquidity slugs, we call _clearPositions which closes the liquidity slugs and withdraws tokens, then _update creates new slugs and transfers tokens back to PoolManager

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With flash accounting, we should be able to leave outstanding deltas -- token credits (from _clearPositions) can be used towards _update

this should might save 120k gas -- 30k gas per token transfer (2x withdraws and 2x "redeposits")

[clemlak]

Saving this for a later release of Doppler.