Network support for bi-directional cursors

[dyemanov]

This ticket complements #1189. The original implementation of bi-directional (aka scrollable) cursors covered only the engine and API, thus being limited to PSQL and embedded connections using DSQL. The complete implementation, however, should also support scrollable cursors for remote connections as well. The remote protocol needs to be extended for that.

[dyemanov]

Implementation fully supports the packet prefetch logic existed before. However, it's enabled only for fetchNext() and fetchPrior() operations, other scrolling options discard the priorly prefetched rows (if any) and switch to fetching one-by-one. There should be no performance difference in uni-directional fetching between regular and scrollable cursors, except the server-side caching cost for scrollable cursors.

Network protocol changes:

1. op_execute and op_execute2 packets are extended to include cursor flags (LONG), this corresponds to the last parameter in openCursor() API method.

if (port->port_protocol >= PROTOCOL_FETCH_SCROLL)
	MAP(xdr_u_long, sqldata->p_sqldata_cursor_flags);

2. New packet op_fetch_scroll is introduced. It mirrors op_fetch with two additional fields added - fetch operation (SHORT) and fetch position (LONG). Possible fetch operations are: NEXT, PRIOR, FIRST, LAST, ABSOLUTE, RELATIVE.

enum P_FETCH
{
	fetch_next = 0,
	fetch_prior = 1,
	fetch_first = 2,
	fetch_last = 3,
	fetch_absolute = 4,
	fetch_relative = 5
};

if (p->p_operation == op_fetch_scroll)
{
	MAP(xdr_short, reinterpret_cast<SSHORT&>(sqldata->p_sqldata_fetch_op));
	MAP(xdr_long, sqldata->p_sqldata_fetch_pos);
}

op_fetch packet may still be used for fetching forward even if the cursor is open as scrollable. {op_scroll_fetch, NEXT} may be used for fetching forward even if the cursor is open as non-scrollable.

[pavel-zotov]

Test will be added after migration to pytest/firebird-driver framework.

[pavel-zotov]

QA note.
Issue is covered by test for #7056 (see notes there about remote protocol; fix was pushed in 5.0.0.320, commit #5a5a2992, 26-nov-2021 09:11)