fix(pg-native): send Buffer parameters as bytea hex so no byte is lost

[nigrosimone]

Fixes #980. libpq gets every parameter as a C string, so a Buffer is read as utf8 and cut at its first zero byte: SELECT $1::bytea with dd 56 00 33 00 returns ef bf bd 56. pg sends the same Buffer in the binary format and returns it intact.

pg-native now turns a Buffer into the hex form bytea accepts, \x..., before handing the values to libpq, at every call that takes parameters: query, execute, the sync forms and the pipelined batch. Sending it in the binary format would need node-libpq to take a format per parameter, which it does not.

The added test sends a Buffer with a zero byte through a query, a prepared statement and the sync forms: it fails on master and passes here.

[nigrosimone]

Closing: the hex form is right for a bytea column but wrong for a text one, and the test for #675 inserts a Buffer into a TEXT column and expects the text. Without a binary parameter format from node-libpq the two cannot both work, so the fix belongs in node-libpq.