Convert schema_translate to a post compile

Convert schema_translate to a post compile

Revised the :paramref:`.Connection.execution_options.schema_translate_map`
feature such that the processing of the SQL statement to receive a specific
schema name occurs within the execution phase of the statement, rather than
at the compile phase.   This is to support the statement being efficiently
cached.   Previously, the current schema being rendered into the statement
for a particular run would be considered as part of the cache key itself,
meaning that for a run against hundreds of schemas, there would be hundreds
of cache keys, rendering the cache much less performant.  The new behavior
is that the rendering is done in a similar  manner as the "post compile"
rendering added in 1.4 as part of :ticket:`4645`, :ticket:`4808`.

Fixes: #5004
Change-Id: Ia5c89eb27cc8dc2c5b8e76d6c07c46290a7901b6