document autosave

[davecramer]

New connection property: autosave

conservative (default) -- rollback to savepoint only in case of "prepared statement does not exist" and "cached plan must not change result type". Then the driver would re-execute the statement ant it would pass through
never -- never set automatic safepoint. Note: in this mode statements might still fail with "cached plan must not change result type" in autoCommit=FALSE mode
always -- always rollback to "before statement execution" state in case of failure. This mode prevents "current transaction aborted" errors. It is similar to psql's ON_ERROR_ROLLBACK.
For autocommit=true the savepoint is not used.
In conservative mode, savepoint is used only for queries that return something.