[SYCL] Guard access to memory objects with a mutex (#5231)

* Protected access to all the non-const member variables of pi_mem objects.
* Added new env variable SYCL_PI_LEVEL_ZERO_SINGLE_THREAD_MODE which
allows to enable single-threaded mode to avoid mutex locking overhead in the L0 plugin.

Author: againull

sycl/doc/EnvironmentVariables.md

@@ -140,6 +140,7 @@ variables in production code.</span>
 
 | Environment variable | Values | Description |
 | -------------------- | ------ | ----------- |
+| `SYCL_PI_LEVEL_ZERO_SINGLE_THREAD_MODE` | Integer | A single-threaded app has an opportunity to enable this mode to avoid overhead from mutex locking in the Level Zero plugin. A value greater than 0 enables single thread mode. A value of 0 disables single thread mode. The default is 0. |
 | `SYCL_PI_LEVEL_ZERO_MAX_COMMAND_LIST_CACHE` | Positive integer | Maximum number of oneAPI Level Zero Command lists that can be allocated with no reuse before throwing an "out of resources" error. Default is 20000, threshold may be increased based on resource availabilty and workload demand. |
 | `SYCL_PI_LEVEL_ZERO_USM_ALLOCATOR` | [EnableBuffers][;[MaxPoolSize][;[host\|device\|shared:][MaxPoolableSize][,[Capacity][,SlabMinSize]]]...] | EnableBuffers enables pooling for SYCL buffers, default 0, set to 1 to enable. MaxPoolSize is the maximum size of the pool, default 0. MemType is host, device or shared. Other parameters are values specified as positive integers with optional K, M or G suffix. MaxPoolableSize is the maximum allocation size that may be pooled, default 0 for host and shared, 32KB for device. Capacity is the number of allocations in each size range freed by the program but retained in the pool for reallocation, default 0. Size ranges follow this pattern: 64, 96, 128, 192, and so on, i.e., powers of 2, with one range in between. SlabMinSize is the minimum allocation size, 64KB for host and device, 2MB for shared. Example: SYCL_PI_LEVEL_ZERO_USM_ALLOCATOR=1;32M;host:1M,4,64K;device:1M,4,64K;shared:0,0,2M|
 | `SYCL_PI_LEVEL_ZERO_BATCH_SIZE` | Integer | Sets a preferred number of compute commands to batch into a command list before executing the command list. A value of 0 causes the batch size to be adjusted dynamically. A value greater than 0 specifies fixed size batching, with the batch size set to the specified value. The default is 0. |