Fix for casting tdigest to text and vice versa

[thanodnl]

As discussed; we ran into an issue when you cast a tdigest to text and later back to tdigest.

The char *centroids would only contain the data till the first word break. Instead we introduce the use of %n to count the length of the data captured by the header. This gets added to the char *str and captured in ptr before parsing the centroids on their own.

While working on this patch I also ran into pq_getmsgint(buf, sizeof(int64)); being called. According to the documentation in pqformat.c Postgres decided to not implement the 64 bit variant:

/* --------------------------------
 *		pq_getmsgint64	- get a binary 8-byte int from a message buffer
 *
 * It is tempting to merge this with pq_getmsgint, but we'd have to make the
 * result int64 for all data widths --- that could be a big performance
 * hit on machines where int64 isn't efficient.
 * --------------------------------
 */

I think this gets used when you store a tdigest in a relation, so I also added a test at the end that will store some tdigest data in a table that gets combined into the final tdigest before verifying the correctness of the results.

[tvondra]

Thanks. I've squashed the commits and merged it like that.

Nice catch with the pg_getmsgint call. I wonder if we might test that somehow (the existing tests only check the text format, I think), but the only way I can think of is COPY with (FORMAT binary), which does not seem particularly nice in regression tests.