fix[N08] Propose fixes to some naming issues

[chrismaree]

Problem:

We have identified some areas of the code which could benefit from better naming:

• In HubPoolInterface.liquidityUtilizationPostRelay, the parameter token should be
  renamed to l1Token to better match other functions in the interface, as well as the function's
  implementation in HubPool.
• In the RootBundle struct, requestExpirationTimestamp should be renamed to better indicate
  that it ends the "challenge period". Consider renaming it to ChallengePeriodEndTimestamp or
  similar.
• The RootBundleExecuted event in HubPool.sol only names one of its array parameters in
  the plural form, but when the event is emitted, all array parameters are named in the plural
  form. Consider changing the event definition so that all array parameters are pluralized.
• The name of function whitelistedRoute is vague and does not indicate what it's output will
  be. Consider renaming it to something like destinationTokenFromRoute to better match the
  return value.
• When weth is used in Polygon_SpokePool.sol, it refers to wrapped MATIC. Consider renaming
  the weth variable in SpokePool.sol to wrapped_native_token to make it more generalizable.
• This will make Polygon_SpokePool less confusing and be more generalizeable for future
  SpokePools.
• The executeSlowRelayRoot and executeRelayerRefundRoot functions are executing leaves
  and should be renamed accordingly.
• The unclaimedPoolRebalanceLeafCount parameter of the ProposeRootBundle event should be
  renamed to poolRebalanceLeafCount, since it's always the total number of leaves in the tree.
• The RootBundleCanceled event names the last parameter as disputedAncillaryData, but the
  proposal is not necessarily disputed. It should just be ancillaryData.
• The _append function of the LpTokenFactory could be called _concatenate to better describe
  its functionality.
• The onlyEnabledRoute modifier has a destinationId parameter that should be
  destinationChainId to match the rest of the code base.

Solution:

all recommendations have been made.

[chrismaree]

Note that this PR requires a few others to go in first that it is dependent on to avoid conflicts. In particular:

• fix[N02] Move all structs to the same place #100
• fix[N03] Fixed inconsistant token metadata versioning #101

[chrismaree]

this change is potentially contentious. I'm curious what you think.

[chrismaree]

unrelated but saw this comment issue.

[chrismaree]

I'm not sure why it shows this diff here...previous PR moved this around.

[nicholaspai]

You can remove this change since ancillaryData is no longer included in this event following #114

[nicholaspai]

This looks awesome but rebase master before you merge and check on any merge conflicts