[explorer] fix DataTables warning about Reinitialise DataTable

Summary: I think the error should be due to the DataTable initialization in blocks.js attempting to reinitialize during certain conditions (such as cache conflicts caused by rapid clicking). Adding retrieve: true should be a choice without negative side effects. Since it hasn't been tested in an instance, I hope the explorer dev could conduct a test.

Test Plan:
It has only been tested in the browser's console.

before:

console.log("Initializing DataTable...");
$('#blocks-table').DataTable({
});

after:
console.log("Initializing DataTable...");
$('#blocks-table').DataTable({

retrieve: true,

});

Reviewers: bytesofman, emack, O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14994