Don't append meshms timestamps if the clock is considered unreliable

conf_schema.h

@@ -417,12 +417,12 @@ END_STRUCT
 STRUCT(rhizome)
 ATOM(bool_t,                enable,         1, boolean,, "If true, server opens Rhizome database when starting")
 ATOM(bool_t,                fetch,          1, boolean,, "If false, no new bundles will be fetched from peers")
+ATOM(bool_t,                reliable_clock, 1, boolean,, "If false, consider the system clock to be unreliable for generating timestamps")
 ATOM(bool_t,                clean_on_open,  0, boolean,, "If true, Rhizome database is cleaned at start of every command")
 STRING(256,                 datastore_path, "", str_nonempty,, "Path of rhizome storage directory, absolute or relative to instance directory")
 ATOM(uint64_t,              database_size,  UINT64_MAX, uint64_scaled,, "Maximum size of database in bytes")
 ATOM(uint64_t,              min_free_space, 100*1024*1024, uint64_scaled,, "Minimum free space to preserve on the disk")
 ATOM(uint32_t,              max_blob_size,  128 * 1024, uint32_scaled,, "Store payloads larger than this in files not SQLite blobs")
-
 ATOM(uint64_t,              idle_timeout,           RHIZOME_IDLE_TIMEOUT, uint64_scaled,, "Rhizome transfer timeout if no data received.")
 ATOM(uint32_t,              fetch_delay_ms,         50, uint32_nonzero,, "Delay from receiving first bundle advert to initiating fetch")
 SUB_STRUCT(rhizome_direct,  direct,)

meshms.c

@@ -218,6 +218,8 @@ static size_t append_footer(unsigned char *buffer, char type, size_t message_len
 // append a timestamp as a uint32_t with 1s precision
 static size_t append_timestamp(uint8_t *buffer)
 {
+  if (!config.rhizome.reliable_clock)
+    return 0;
   write_uint32(buffer, gettime());
   size_t ofs=4;
   return ofs+append_footer(buffer+ofs, MESHMS_BLOCK_TYPE_TIME, ofs);