The LIKE operators now use an explicit ESCAPE '\' clause for similar behavior across databases

Database behavior for LIKE escaping varies.  PostgreSQL, MySQL,
and H2 seem to use a backslash escape by default, but most other
databases require an explicit escape.  All tested databases seem
to support the ESCAPE clause except Access, so just enable it
by default except on Access.

This is backwards incompatible for adapters where backslashes
are not used as LIKE escapes by default and you are using
a backslash in a LIKE patterns as a non-metacharacter.  The
workaround is to double the backslash in the pattern.
Add Dataset#escape_like, for escaping LIKE metacharacters (Fixes #614)

This is designed for the case where part of a LIKE term is based on
user input, where you want to escape that part, and then add
metacharacters to the beginning or ending of the string.
Make ilike use UPPER and LIKE instead of ILIKE by default

ILIKE is a PostgreSQL extension, so Sequel shouldn't default to
using it.  All other adapters previously override the handling
for ilike.  Most of them used UPPER in some form, so just make
that the default and DRY up some adapter code.

Remove the like_uses_escape? method added recently, and just
always use ESCAPE.  The only database that didn't support this
was access, so just override the shared access adapter to not
use ESCAPE.

This shouldn't an effect for databases that Sequel currently
supports.  For databases without special support, this should
make ILIKE work by default.