New InternalStatistics
collectd.conf: Fix default packet size of the network plugin.

Fixes: #710
Statsd: support samplerate field in timing metric types

Fixes "Unable to parse line" bug, often seen receiving stats from
OpenStack Swift. Should fix GH issue #461
allow to configure the maximum effective interval between attempts to read data by plugins
EL <6 provide perl-ExtUtils-Embed with base perl

Conflicts:
	contrib/redhat/collectd.spec
rename option MaxEffectiveInterval -> MaxReadInterval
nb_values missing in types.db; renaming as cache_size
document MaxReadInterval option
renamed the option to CollectInternalStats; renamed 'internal' to 'collectd'; no more lock when not recording internal stats
collectd.conf(5): Improve documentation of LoadPlugin.

Mention AutoLoadPlugin in the relevant places. Add a <Plugin /> block
for the "df" plugin to make it easier to spot that it is preceded by a
<LoadPlugin /> *block*.

Fixes: #715
collectd.conf(5): Improve AutoLoadPlugin documentation.

Make it clear that plugins without a <Plugin /> block will still need a
LoadPlugin line.

References: #715
java plugin: Make sure cjni_thread_detach() is called on all paths.

Each call to cjni_thread_attach() much be accompanied by
a call to cjni_thread_detach(). Some error handling cases were missing
the call, potentially locking the plugin up.
GenericJMXConfConnection: Remove unused imports.
RPM specfile: update changelog without incrementing release number
Merge branch 'collectd-5.3' into collectd-5.4

Conflicts:
	contrib/redhat/collectd.spec
parse option MaxReadInterval in the plugin_init_all()
src/utils_rrdcreate.c: Free "args".

Thanks to Yves Mettier for pointing this out!

Fixes: #661
python plugin: Don't create empty meta_data_t objects.

Fixes: #716
src/utils_format_json.c: Handle empty meta_data_t gracefully.

Previously, `meta != NULL` and `keys_num == 0` was possible and would
result in "{}", which is not valid JSON.

Fixes: 716
src/meta_data.c: If the meta_data_t is empty, avoid calling malloc(0).