Add to Gift Aid batch timing out

[deepak-srivastava]

Add to Gift Aid batch starts timing out for contributions greater than 3500. Some of our clients got contributions in the range 5k - 14k per month, where it really fails to respond.
On investigation we found that it's in fact the validation process which is timing out.

About to do a PR which converts the validation process into civi queue process, with a progress bar.

[artfulrobot]

Nb. the code that does this is very inefficient, which is probably causing this problem. For every contribution it loads (or reloads in the case of multiple contributions for a given contact) all the declarations for the contact, then php ambles through them one by one seeing if it can find a match. This could all be done much more efficiently with SQL.