Implemented utility to work with Request parameters

[hurynovich]

Overview

This PR is a proposal, which adds utility classes to simplify implementation of Commands.
Two main ideas can be distinguished:

• CommandException - exception which automatically converted into ErrorRedisToken.
• ParamsScanner - utility to simplify parameters parsing for Redis queries. And it actively throws CommandException.

Example

Parsing Redis command:

HSCAN key cursor [MATCH pattern] [COUNT count]

@Command("hscan")
public class HashScanCommand implements DBCommand {
  public static final int DEFAULT_COUNT = 100;
  public static final String DEFAULT_PATTERN = "*";

  @Override
  public RedisToken execute(Database db, Request request) {
    ParamsScanner parser = new ParamsScanner(request);
    String key = parser.nextString();
    int cursor = parser.nextInt();
    GlobPattern pattern = parser.nextGlob("match", DEFAULT_PATTERN);
    int count = parser.nextInt("count", DEFAULT_COUNT);
    parser.verifyHasNoMore();

Motivation

I tried to implement few Redis commands with bunch of optional parameters.
And code contains a lot of boiler plate if/else statements when works with raw request.getParams().
So, at the end I come to utility class which intended to make parameter parsing more concise and readable at the same time.

Note

It is currently a draft.
Tests, naming and docs are not final yet.

Related

#230