Charge gas for custom event attributes and messages

[alpe]

• Resolves Let contracts not abuse the event system #280
• Resolves Limit init/migrate/execute payload message size #203

Features

• Introduces GasRegister and extension point
• Moved event emitting and charging gas into response handler method

[ethanfrey]

Nice start.
Added some comments.

What do you think of a price per attribute + price per byte (like iavl storage access)?

[codecov]

Codecov Report

Merging #539 (f9521eb) into master (6a6d6c9) will increase coverage by 0.15%.
The diff coverage is 94.57%.

@@            Coverage Diff             @@
##           master     #539      +/-   ##
==========================================
+ Coverage   58.51%   58.67%   +0.15%     
==========================================
  Files          43       44       +1     
  Lines        4532     4554      +22     
==========================================
+ Hits         2652     2672      +20     
- Misses       1662     1663       +1     
- Partials      218      219       +1     

Impacted Files	Coverage Δ
x/wasm/keeper/msg_server.go	0.00% <0.00%> (ø)
x/wasm/types/types.go	51.35% <ø> (ø)
x/wasm/keeper/gas_register.go	96.00% <96.00%> (ø)
x/wasm/keeper/keeper.go	84.29% <100.00%> (-0.21%)	⬇️
x/wasm/keeper/options.go	72.50% <100.00%> (-3.59%)	⬇️
x/wasm/keeper/proposal_handler.go	68.42% <100.00%> (ø)
x/wasm/keeper/query_plugins.go	77.01% <100.00%> (ø)
x/wasm/keeper/relay.go	100.00% <100.00%> (ø)

[ethanfrey]

Good stuff. I would just like Simon to review the proper max value