ntp: optimize detection of clients using interleaved mode

Use the lowest bit of the server RX and TX timestamp as a flag
indicating RX timestamp. This allows the server to detect potential
interleaved requests without having to save all its RX timestamps. It
significantly reduces the amount of memory needed to support clients
using the interleaved mode if most of the server's clients are using the
basic mode (e.g. a public server).

Capture the TX timestamp on the first response to the request which has
the flag set to not further delay the first interleaved response.

False positives are possible with broken clients which set the origin
timestamp to something else than zero or the server RX or TX timestamp.
This causes an unnecessary RX timestamp to be saved and TX timestamp
captured and saved.

doc/chrony.conf.adoc

@@ -1494,9 +1494,11 @@ directive.
 This directive specifies the maximum amount of memory that *chronyd* is allowed
 to allocate for logging of client accesses and the state that *chronyd* as an
 NTP server needs to support the interleaved mode for its clients. The default
-limit is 524288 bytes, which is sufficient for monitoring about four thousand
-clients at the same time. The maximum value is 2^32-1 (4 GB) on 32-bit systems
-and 2^35 (32 GB) on 64-bit systems.
+limit is 524288 bytes, which enables monitoring of up to 4096 IP addresses at
+the same time and holding NTP timestamps for up to 4096 clients using the
+interleaved mode (depending on uniformity of their polling interval). The
+maximum value is 2^32-1 (4 GB) on 32-bit systems and 2^35 (32 GB) on 64-bit
+systems.
 +
 An example of the use of this directive is:
 +