Use process warnings for deprecations

[eritbh]

Addresses #1274. Adds process warning emits alongside existing deprecation-related client warn emits. To avoid emitting the same process warning multiple times, uses a wrapper around process.emitWarning that checks if the given warning code has already been used, and if so, skips emitting the warning again.

[eritbh]

Rewrote the rest of the warning descriptions to be consistent with each other. I've elected not to copy these changes to warnings emitted on the client to avoid potentially breaking things, but if it's desired anyway, I can copy my changes to those as well. Please double check my grammar and format.

[eritbh]

Merged latest dev in again and this branch still appears to work fine for me, though I'd guess there are additional deprecated behaviors that have been introduced in the meantime which will need to be handled in a successive PR.