Speed up suggestion insertion by using a single transaction

[mchowning]

When adding @-mention support for Gutenberg, I noticed that it was taking a really long time for mentions to appear the first time for a site. Part of this is due to the API call taking 3-6 seconds when there are 1500+ users on the site (a site with only a single user responded in around 300ms).

Most of the delay was due to the time it took for the data to be inserted in the database one entry at a time. On my device, inserting 1500+ suggestions for a site took ~16 seconds after the API call returned the data. This PR wraps all those insertions into a single transaction which reduced the insertion time down to under 300 milliseconds in my tests.

This change is important for Gutenberg because we are going to be using this for a UI that is only used for providing suggestions. Having that UI not work for >20 seconds the first time it is loaded is obviously an awful user experience.

Testing

1. Switch to a site you have not used on your device before (or wipe the app data).
2. Open the comments for that site
3. Tap on a comment
4. Tap on the field for replying to a comment
5. Note that the suggestions API call is initiated and that once it completes you almost immediately (< 300ms) see autocomplete suggestions when you type @ immediately followed by a single letter (until that point there won't be any suggestions available). Given how long the API call itself can take, I would suggest watching it using Stetho, Charles, or adding a log message to SuggestionService::handleSuggestionsUpdatedResponse so you know exactly when suggestions should be available.

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading the APK here.

[oguzkocer]

We probably should be doing this in a single query instead of a transaction. I think this is fine as a quick patch though, so

P.S: It's very hard to look at this code for me, it's from before I switch to Android development 🙈