CA-134765: Fix race condition in database layer

The read_records_where function in the database layer (used by the
get_all_records and get_all_records_where APIs) was reading the database
multiple times by calling find_refs_with_filter[1] to get the refs that matched
the query and then calling read_record[2] for each of these refs.

This violates point #2 of the  locking strategy stated at the top of the module
that read only functions must only call get_database once to ensure they
operate on a consistent snapshot.

Since [1] and [2] make get_database calls the get_all_records* functions make
n+1 calls to get_database for a table with n records. Because of this, deleting
a record during a get_all_records_where will result in a DBCache_NotFound
exception with parameter "missing row".

This commits adds internal variants of functions [1] and [2] that take
an actual instance of Database.t rather than a Db_ref.t which is a Database.t
ref ref.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Merge pull request #1746 from simonjbeaumont/ca-134765-creedence

[xs64bit] CA-134765: Fix race condition in database layer