thread-safety fixes: PooledString, layer metadata

bug 1: PooledString resizes `vector` without locks

`tables` is a shared pool of `char*` pointers, where each pointer
points to a 64KB memory chunk.

Some `PooledString`s identify their content by an index into this pool.

However, `tables` can grow. We correctly guard against concurrent
mutation (for example, here:
https://github.com/systemed/tilemaker/blob/7f0343045687ab2125910c81eed598c58fc2ff2d/src/pooled_string.cpp#L33-L39)

But readers expect to be able to read it without a lock, for example
here, where the result of a read will be used to do a write: https://github.com/systemed/tilemaker/blob/7f0343045687ab2125910c81eed598c58fc2ff2d/src/pooled_string.cpp#L54

This pattern isn't safe with `vector`, since when the `vector` grows,
it invalidates all existing pointers. It is safe with `deque`, so the fix
is to switch to a `deque`.

bug 2: vector layer metadata `map` isn't guarded

`layers` is a shared object common to all OsmLuaProcessing threads.

`layers.layers` is a `vector` that gets initialized and populated fully on
the main thread before the Lua threads start, so accessing it without
locks is fine.

`layers.layers[layer].attributeMap` is just a vanilla `map`, though,
so mutating it from multiple threads without locks is dangerous.

I just added a coarse lock for now. On my 16-core machine, it didn't
seem to introduce contention, so I didn't bother to do anything fancier
to minimize locking overhead.

I will optimistically say that this fixes systemed#746.

src/osm_lua_processing.cpp

@@ -17,6 +17,8 @@ const std::string EMPTY_STRING = "";
 thread_local kaguya::State *g_luaState = nullptr;
 thread_local OsmLuaProcessing* osmLuaProcessing = nullptr;
 
+std::mutex vectorLayerMetadataMutex;
+
 void handleOsmLuaProcessingUserSignal(int signum) {
 	osmLuaProcessing->handleUserSignal(signum);
 }
@@ -913,6 +915,7 @@ std::string OsmLuaProcessing::FindInRelation(const std::string &key) {
 
 // Record attribute name/type for vector_layers table
 void OsmLuaProcessing::setVectorLayerMetadata(const uint_least8_t layer, const string &key, const uint type) {
+	std::lock_guard<std::mutex> lock(vectorLayerMetadataMutex);
 	layers.layers[layer].attributeMap[key] = type;
 }
 

src/pooled_string.cpp

@@ -2,9 +2,10 @@
 #include <stdexcept>
 #include <mutex>
 #include <cstring>
+#include <deque>
 
 namespace PooledStringNS {
-	std::vector<char*> tables;
+	std::deque<char*> tables;
 	std::mutex mutex;
 
 	const uint8_t ShortString = 0b00;