[SPARK-13890][SQL] Remove some internal classes' dependency on SQLContext

## What changes were proposed in this pull request?
In general it is better for internal classes to not depend on the external class (in this case SQLContext) to reduce coupling between user-facing APIs and the internal implementations. This patch removes SQLContext dependency from some internal classes such as SparkPlanner, SparkOptimizer.

As part of this patch, I also removed the following internal methods from SQLContext:
```
protected[sql] def functionRegistry: FunctionRegistry
protected[sql] def optimizer: Optimizer
protected[sql] def sqlParser: ParserInterface
protected[sql] def planner: SparkPlanner
protected[sql] def continuousQueryManager
protected[sql] def prepareForExecution: RuleExecutor[SparkPlan]
```

## How was this patch tested?
Existing unit/integration tests.

Author: Reynold Xin <rxin@databricks.com>

Closes #11712 from rxin/sqlContext-planner.