Passing an already canceled/expired context to QueryContext/ExecContext
would return empty results without error because interruptOnDone would
immediately set the "done" flag to one so the calling function (query or
exec) would not enter the for loop doing the meaningful work.

Similarly, if the context is expired/canceled at the "wrong time", the
same behavior could happen.

This patch first check the context is still valid when entering the
function (before launching interruptOnDone), then if the funxtion is
about to return no values and no error, it checks if the "done" flag is
set and return the context error if it makes sense.

Handle context expired/canceled in Query/ExecContext

See merge request cznic/sqlite!65

While the query had succeeded, the context can expire right after. In
this situation, iterating the rows will fail. The test need to check for
sql.Rows.Err() too.

Also, don't forget to close sql.Rows when done to avoid memory leak.

fix error handling in test "QueryContext with context expiring"

See merge request cznic/sqlite!66

[bugfix] ensure statement pointers finalized

Closes #178

See merge request cznic/sqlite!67

docs(README): add `database is locked` error to new "troubleshooting" section

See merge request cznic/sqlite!69

feat: 89: Add Backup.Commit method

See merge request cznic/sqlite!70

TIL that 'go get example.com/foo' downloads only what the respective
package requires, but 'go mod tidy' will also download test-only
dependencies.

If needed, the CGo dependencies can be revived in a separate
repository/module. For now just delete it.

h/t: Dan Kortschak

use passed context in BeginTx

See merge request cznic/sqlite!71