Fix list-all-zones in multi-backend configuration involving LMDB

[miodvallat]

Short description

I noticed recently that a multi-backend configuration with LMDB as the last backend would not display all domains in pdnsutil list-all-zones (and possibly other operations).

This is caused by the LMDB backend replacing the data instead of appending to it.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[coveralls]

Pull Request Test Coverage Report for Build 13591519276

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 43 unchanged lines in 10 files lost coverage.
• Overall coverage increased (+0.02%) to 64.503%

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Files with Coverage Reduction	New Missed Lines	%
pdns/pollmplexer.cc	1	83.66%
pdns/distributor.hh	2	51.86%
pdns/tcpiohandler.cc	2	68.18%
pdns/ws-auth.cc	2	80.79%
modules/gpgsqlbackend/spgsql.cc	3	67.7%
pdns/recursordist/test-syncres_cc2.cc	3	88.91%
pdns/recursordist/test-syncres_cc1.cc	5	89.91%
pdns/dnsdistdist/dnsdist-carbon.cc	6	62.5%
pdns/recursordist/syncres.cc	7	80.14%
ext/json11/json11.cpp	12	62.72%

Totals
Change from base Build 13588225536:	0.02%
Covered Lines:	127657
Relevant Lines:	166926

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💛 - Coveralls