maint: change default for MaxSendMsgSize and MaxRcvMsgSize.

[kentquirk]

Which problem is this PR solving?

• Fixes Discussion: Raising the max receive message size default for GRPC connections? #1131
• The default receive message size is smaller than Honeycomb accepts, which can be confusing. Change it, and the default send size, for better efficiency.

Short description of the changes

• Change the defaults from 5MB to 15MB
• This is technically a breaking change but only by the most pedantic definition -- there's no realistic situation in which this actually matters, so we're not going to stress over it.

[tdarwin]

[VinozzZ]

I think we should also change the default value in the file_config.go file so that they are consistent