Add spec for the dynamic configuration.

specs/dynamic_configuration.rst

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+Patroni configuration reload
+============================
+
+Patroni configuration should be stored in the DCS. There will be 3 types of configuration:
+
+- default configuraton set in Patroni
+	That should be applied during the initialization time and written to etcd.
+
+- startup configuration (also in patroni.yml).
+	They should be applied during the initialization time. Unlike other options, they are not written in etcd and
+	any attempts to change them dynamically are blocked.
+
+- dynamic configuration.
+	Those options can be set in etcd at any time. If the options changed are not part of the startup configuration,
+	they are applied asynchronously (upon the next wake up cycle) to every node, which gets subsequently reloaded.
+	If the node requires a restart to apply the configuration (for options with context postmaster or internal, if
+	their values have changed), a special flag indiciating this should be set in the members.data JSON. A new API
+	endpoint should return whether the given node requires a restart. Additionally, the node status shoudl also 
+	idincate this.
+
+Some options may not be increased on the master independently of the replicas (master-dependent options):
+
+- max_connections
+- max_locks_per_transactions
+- max_worker_processes
+- max_prepared_transactions
+
+Regarding the options that can be set, the following restrictions apply:
+- dynamic configuration options that are also listed in the startup configuration will not be changed, except for the case
+of master-dependent options.
+
+When applying the startup or dynamic configuration options, the following actions should be taken:
+- The node should first check if there is a postgresql.conf.patroni.
+- If it exists, it contains the renamed "original" configuration.
+- If it doesn't, the original postgresql.conf is taken and renamed to postgresql.conf.patroni.
+- The dynamic options (with the exceptions above) are dumped into the postgresql.conf and an include is set in
+postgresql.conf to postgresql.conf.patroni. Therefore, we woul be able to apply new options without re-reading the conf file
+to check if the include is present not.
+- If some of the options that require restart are changed (we should look at the context in pg_settings and at the actual
+values of those options), a restart_pending flag of a given node should be set. This flag is reset on any restart.
+
+Also, the following patroni configuration options can be changed dynamically:
+
+- ttl
+- loop_wait
+- retry_timeouts (to be defined first in patroni.yaml)
+
+Upon changing those options, Patroni should read the relevant section of the configuration storedi in DCS and change their
+run-time values.
+
+Patroni nodes should dump the state of the DCS options to disk on startup and upon every change of the configuration.
+Only master is allowed to restore those options from the on-disk dump if those are completely absent from the DCS on invalid.
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