lib-sql: Use generic sql connection pooling code for mysql/pgsql.

lib-sql: Use generic sql connection pooling code for mysql/pgsql.
It's possible to give multiple host settings to do load balancing / HA.
If one host is down, another one is tried. All queries are automatically
retried in another host if they fail in first one.

Since PostgreSQL support async queries, Dovecot can create multiple
connections to the database as needed, so it can do lookups in parallel. The
number of connections can be changed with maxconns=n in connect_query, the
default is 5.

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branch : HEAD