Add sangria-slowlog middleware to allow for GraphQL slow logging

[cliu587]

cc @dguo-coursera, @yifan-coursera

Use https://github.com/sangria-graphql/sangria-slowlog as right now it's impossible to understand what's making my queries take a long time.

Any idea how to test this change with assembler locally?

[cliu587]

Also:

1. I realize you have a similar change https://github.com/coursera/naptime/pull/222/files
2. I can't seem to compile Naptime even on master using sbt. Getting

[error] /Users/cliu/base/coursera/naptime/naptime-graphql/src/main/scala/org/coursera/naptime/ari/graphql/controllers/GraphQLController.scala:177: missing parameter type for expanded function
[error] The argument types of an anonymous function must be fully known. (SLS 8.5)
[error] Expected type was: sangria.execution.Executor.ExceptionHandler
[error]   def exceptionHandler(logger: Logger): Executor.ExceptionHandler = {
[error]                                                                     ^
[error] /Users/cliu/base/coursera/naptime/naptime-graphql/src/main/scala/org/coursera/naptime/ari/graphql/types/NaptimeTypes.scala:33: wrong number of arguments for pattern sangria.ast.StringValue(value: String,block: Boolean,blockRawValue: Option[String],comments: Vector[sangria.ast.Comment],location: Option[sangria.ast.AstLocation])
[error] Error occurred in an application involving default arguments.
[error]       case StringValue(string, _, _) => stringToDataMapEither(string)
[error]                       ^
...

[cliu587]

Closing this because I accidently amended my commit locally. Will create a new PR soon.