[bugfix] add synth units to pools

Son of Odinrequested to merge
synth-fix into develop

Fixes #795 (closed)

Adding the concept of "synth units" to pools. This is basically LP units but held by the pool to represent the amount of synth asset a pool has. This is to ensure that the rune added/removed from the pool does not leave the pool via LP withdrawal. Instead, the pool holds that rune until a synth asset is swapped out to rune.

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