By continuing to serve the configuration database interface used by
clients even when disabled, clients no longer need to specify a special
`--no-config-db` option when running commands which talk to the
configuration database.
The `clearknob` command clears the value that a knob has been set to in
the configuration database. Note that this does not mean the knob value
itself gets cleared - only the value in the configuration database is
cleared. The value of the knob will revert to whatever is hardcoded in
the corresponding `*Knobs.cpp` file.
Sample `fdbcli` session:
```
Welcome to the fdbcli. For help, type `help'.
fdb> getknob min_trace_severity
`min_trace_severity' is not found
fdb> setknob min_trace_severity 20
Please set a description for the change. Description must be non-empty
description: test
Committed (2)
fdb> getknob min_trace_severity
`min_trace_severity' is `20'
fdb> clearknob min_trace_severity
Please set a description for the change. Description must be non-empty
description: clear
Committed (4)
fdb> getknob min_trace_severity
`min_trace_severity' is not found
```
Transactions are also supported with the new `clearknob` command:
```
Welcome to the fdbcli. For help, type `help'.
fdb> begin
Transaction started
fdb> setknob min_trace_severity 20
fdb> clearknob min_trace_severity
fdb> commit
Please set a description for the change. Description must be non-empty.
description: test
Committed (16)
fdb> getknob min_trace_severity
`min_trace_severity' is not found
```
