refactor: replace"vested" terminology with "locked"

[RiccardoM]

Description

This PR updates the x/liquidvesting module to replace the "vested" terminology with "locked" instead. This is to make it clearer that we are representing vesting (i.e. not yet vested, still locked) tokens rather than vested (i.e. which have completed the vesting period, unlocked) tokens.

Depends-On: #204

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