add display_prices in variation data

The commit introduces 3 changes:
1. It adds 2 extra fields to the available_variations array,
display_price and display_regular_price, to be used by extensions in
cases like
https://github.com/woothemes/woocommerce-product-addons/issues/19
2. Introduces a get_display_price() function in WC_Product to retrieve
product prices incl or excl tax relative to the
woocommerce_tax_display_shop setting.
3. Fixes a minor issue where passing a 0 price to
get_price_including_tax() or get_price_excluding_tax() results in a
wrong output.
Added the possibility to display notices and/or force reload on order review.
Delete cached sessions when clearing expired sessions
Fixes #6907
only delete sessions from object cache if using persistent object cache
ajaxified coupon removal on checkout page for better UX closes #6881
Merge pull request #6911 from splashingpixels/checkout-coupons

ajaxified coupon removal on checkout page for better UX closes #6881
Merge pull request #6909 from webaware/cleanup-sessions-cached

Delete cached sessions when clearing expired sessions
Retrive reviewers avatar using the email

Using the email will fix the wrong avatar issue occurring while creating manually the review from the admin panel.
Correctly save custom address fields in admin
Error in previous commit.
Merge pull request #6914 from SiR-DanieL/patch-2

Retrieve reviewers avatar using the email
Escape `_` in LIKE to allow index search

Also filters for expired sessions in DB, not PHP.
Fixes #6916
Don't return option_value from cleanup_sessions() query, no longer required
woocommerce_price_filter_meta_keys filter to allow plugins to define min/max meta keys that require searching

Closes #6492
Leave postcode before city swapping to JS

Fixes #6842
Fix possible notice
Fix admin term output

Resolves an issue decoding foreign chars caused by #6555
Fixes #6550
Add special wc_query variable when querying products. #6444
Merge pull request #6920 from SiR-DanieL/patch-2

Fix possible notice with taxes
Merge pull request #6917 from webaware/cleanup-sessions-escape-for-like

Escape `_` in LIKE to allow index search