CDAP-538: Ported procedures of WordCount app to services.

@ParthGandhi The WordCount app has been updated with services instead of procedures, please take a look.
\3. A Service named \`\`RetrieveCounts\`\` serves read requests for calculated statistics,
word counts and associations. It exposes these endpoints:

\- \`\`/stats\`\` returns the total number of words, the number of unique words, and the average word length;
\- \`\`/count/{word}/{limit}\`\` returns the word count of a specified word and its word associations, up to the specified limit;
\- \`\`/count/{word}\`\` returns the word count of a specified word and its word associations, to a limit of ten; and
\- \`\`/assoc/{word1}/{word2}\`\` accesses the top associated words (those with the highest counts).