lib-sql: Add comments about NULL values.

lib-sql: Fix compiler warning with OSX

lib-sql: Add support for prepared SQL statements. This initial implementation doesn't use prepared statements in drivers, but simply generates the query string internally.

lib-sql: Add API support for asynchronously iterating over rows. sql_query() can already do an async lookup, but the full result needs to be available immediately. This can be inefficient for large results. Add a new SQL_RESULT_NEXT_MORE return value and sql_result_more() for asynchronously requesting more results. This changes the API a bit, but isn't done by default by any drivers yet. Also callers that can't handle this are hopefully checking for "ret < 0", which allows them to handle such an async-more request as an error instead. sql_result_next_row() will be changed to return enum in a separate commit to keep backwards compatibility in v2.2.x.

lib-sql: Added error_type to commit callback.

lib-sql: Added sql_result_get_error_type(). For now the only special error type is SQL_RESULT_ERROR_TYPE_WRITE_UNCERTAIN.

lib-sql: Implemented sql_escape_blob()

Removed CONTEXT_TYPE_SAFETY macro and reimplemented its functionality better. gcc/clang now gives a compiler error in many places if callback isn't exactly what was expected. It's also now much easier to add more of these checks.

lib-sql: sql_disconnect() now aborts all pending requests.

lib-sql: Use generic sql connection pooling code for mysql/pgsql. It's possible to give multiple host settings to do load balancing / HA. If one host is down, another one is tried. All queries are automatically retried in another host if they fail in first one. Since PostgreSQL support async queries, Dovecot can create multiple connections to the database as needed, so it can do lookups in parallel. The number of connections can be changed with maxconns=n in connect_query, the default is 5. --HG-- branch : HEAD

lib-sql: Added sql_update_get_rows(). --HG-- branch : HEAD

SQL API change: SQL results can be now refed/unrefed. --HG-- branch : HEAD

Changed .h ifdef/defines to use <NAME>_H format. --HG-- branch : HEAD

Added sql_result_setup_fetch() which makes it easier to fetch rows into structures. Added sql_result_get_field_value_binary(), which is currently not implemented for MySQL. --HG-- branch : HEAD

Replaced void *context from a lot of callbacks with the actual context type. Also added/fixed some context type checks. --HG-- branch : HEAD

Type safe callbacks weren't as easy as I thought. Only callback(void *context) can be handled generically. Others can be handled specially, but only if all the parameters are pointers, otherwise eg. int parameter can be replaced with long without compiler giving any warnings. --HG-- branch : HEAD

Added context parameter type safety checks for most callback APIs. --HG-- branch : HEAD

sql_escape_string() should return const char *, not char *. --HG-- branch : HEAD

Added sql_escape_string() --HG-- branch : HEAD

Removed extra commas from end of enum lists. --HG-- branch : HEAD

Added support for dynamically building SQL drivers. --HG-- branch : HEAD

deinit, unref, destroy, close, free, etc. functions now take a pointer to their data pointer, and set it to NULL. This makes double-frees less likely to cause security holes. --HG-- branch : HEAD

Added support for transactions and synchronous SQL queries. --HG-- branch : HEAD

s/occured/occurred/ --HG-- branch : HEAD

Added sql_connect() to do explicit connecting. sql_init() no longer does. --HG-- branch : HEAD

Added sql_get_flags() function, currently returning only SQL_DB_FLAG_BLOCKING. --HG-- branch : HEAD

Created generic asynchronous SQL API and implemented MySQL and PostgreSQL drivers. MySQL is implemented synchronously because it's API doesn't provide async way to do it. Replaced pgsql and mysql userdb/passdb with generic sql userdb/passdb. --HG-- branch : HEAD