pgsql: Improvements from Heikki Linnakangas <heikki@enterprisedb.com>

Log Message:
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Improvements from Heikki Linnakangas <heikki(at)enterprisedb(dot)com>

- change the alignment requirement of lexemes in TSVector slightly.
Lexeme strings were always padded to 2-byte aligned length to make sure
that if there's position array (uint16[]) it has the right alignment.
The patch changes that so that the padding is not done when there's no
positions. That makes the storage of tsvectors without positions
slightly more compact.

- added some #include "miscadmin.h" lines I missed in the earlier when I
added calls to check_stack_depth().

- Reimplement the send/recv functions, and added a comment
above them describing the on-wire format. The CRC is now recalculated in
tsquery as well per previous discussion.

Modified Files:
--------------
pgsql/src/backend/utils/adt:
tsginidx.c (r1.2 -> r1.3)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/tsginidx.c?r1=1.2&r2=1.3)
tsquery.c (r1.4 -> r1.5)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/tsquery.c?r1=1.4&r2=1.5)
tsquery_cleanup.c (r1.3 -> r1.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/tsquery_cleanup.c?r1=1.3&r2=1.4)
tsquery_rewrite.c (r1.3 -> r1.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/tsquery_rewrite.c?r1=1.3&r2=1.4)
tsquery_util.c (r1.3 -> r1.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/tsquery_util.c?r1=1.3&r2=1.4)
tsrank.c (r1.3 -> r1.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/tsrank.c?r1=1.3&r2=1.4)
tsvector.c (r1.3 -> r1.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/tsvector.c?r1=1.3&r2=1.4)
tsvector_op.c (r1.3 -> r1.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/tsvector_op.c?r1=1.3&r2=1.4)
pgsql/src/include/tsearch:
ts_type.h (r1.3 -> r1.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/tsearch/ts_type.h?r1=1.3&r2=1.4)