fix(grpc): use Numbers to represent long values

[mrfelton]

Configure gRPC to use Numbers to represent long values instead of Strings. This ensures that we can do accurate handling of numerical values.

Description:

This ensures that numbers are actually numbers and not strings so a check like if (val) { ... } would will give expected results.

For example:

if (0) { echo "something" }
   => undefined

but

if ("0") { echo "something" }
    => "something"

("0" is a truthy value, whilst 0 is not)

Motivation and Context:

Fix #727

Types of changes:

Bux fix

Checklist:

• My code follows the code style of this project.
• I have reviewed and updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes where needed.
• All new and existing tests passed.
• My commits have been squashed into a concise set of changes.

[coveralls]

Coverage remained the same at 15.758% when pulling 47b95a0 on mrfelton:fix/long-as-number into 2931eff on LN-Zap:master.

[JimmyMow]

utACK 47b95a0