Fixed all solidity warnings and applied some best practices. (#1923)

Closes [1913](https://github.com/unionlabs/union/issues/1913)

* Removed all unnecessary variable initializations, unused parameters,
dead code, and fixed function visibilities, etc.
* Some other best practices/gas optimizations applied to code eg: 
  *  Unneeded initializations of an integer variable to `0`.
  *  Cache Local Variable Array Length In For Loop
  *  Unused imports
  *  Conditions can be optimized