pgsql: Rearrange dropdb() to avoid errors after allowing other sessions

Rearrange dropdb() to avoid errors after allowing other sessions to exit.

During Drop Database, it is better to error out before allowing other
sessions to exit and forcefully terminating autovacuum workers.  All the
other errors except for checking subscriptions are already done before.

Author: Amit Kapila
Discussion: https://postgr.es/m/CAA4eK1+qhLkCYG2oy9xug9ur_j=G2wQNRYAyd+-kZfZ1z42pLw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9fab25c6cd1f943284b8e0014007cd5750d54308

Modified Files
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src/backend/commands/dbcommands.c | 26 +++++++++++++-------------
1 file changed, 13 insertions(+), 13 deletions(-)