Add an account key field to improve search performance. This field
allows us to filter the records by account directly in the fulltext
index, so we only need to examine rows belonging to the relevant account.
Lean on ActiveRecord models for searching and strip out the raw SQL.
Replaces the search_index_* tables with sharded search_records_* tables
as that allows us to use a Search::Record model name.
A Class is dynamically created for each record table shard so that we
and we can access it via the Search::Record.for_account(account_id)
method.
Donal McBreen