BREAKING CHANGE!:
1. add session id argument to sql_prepare_ext
2. introduce sql_unprepare_ext function.
This function removes prepared stmt using
given session id.

In picodata SQL, we may prepare stmt in one
session and unprepare it in some
other session, which does not know in
what session the statement was prepared. Now
sql_prepare_ext returns not only statement id,
but also a session id. This way statement can
be unprepared from other session using
sql_unprepare_ext.

NO_DOC=picodata internal patch
NO_CHANGELOG=picodata internal patch