Reword activity message to avoid truncation

Reword activity message to avoid truncation

The message sent to pg_stat_activity when waiting for transactions to
end was long enough that it could get truncated when the xid included
in the string grew large. Fix by rewording to avoid truncation, using
a similar message in the logical replication code as inspiration.

Author: Daniel Gustafsson <daniel@yesql.se>
Reported-by: ChangAo Chen <cca5507@qq.com>
Discussion: https://postgr.es/m/tencent_47CA9CE2F91FF4D81D875E162AD69AA4900A@qq.com