We used to allocate 1024 instances per "concurrency" but we now only
allocate 1024 plus 20 per "concurrency". So, I'm increasing the default
and maximum concurrency factors to take advantage of this. Importantly:

1. Max concurrency 128 -> 4096. `1024 + 4096*20 < 128 * 1024`.
2. Default concurrency 4 -> 150. `1024 + 150*20 < 4 * 1024`.

Motivation: I'm hoping that the default (150) is sufficient (more than
the previous max...) so we can just get rid of this knob.

fixes filecoin-project/lotus#11817

We previously used the FVM's `ThreadedExecutor` to execute messages on
separate threads because the FVM requires 64MiB of stack space.

1. The FVM v3 supported for 8 concurrent threads.
2. The FVM v4 supports up to the number of CPU threads available.

Unfortunately, neither version was influenced by the
`LOTUS_FVM_CONCURRENCY` environment variable.

This patch fixes this by:

1. Moving the thread-pool to the FFI itself (sharing it between FVM
versions).
2. Setting the thread-pool size equal to `LOTUS_FVM_CONCURRENCY`.

It also defaults `LOTUS_FVM_CONCURRENCY` to the number of available
CPU threads instead of the previous 4.

fixes filecoin-project/lotus#11817

We previously used the FVM's `ThreadedExecutor` to execute messages on
separate threads because the FVM requires 64MiB of stack space.

1. The FVM v3 supported for 8 concurrent threads.
2. The FVM v4 supports up to the number of CPU threads available.

Unfortunately, neither version was influenced by the
`LOTUS_FVM_CONCURRENCY` environment variable.

This patch fixes this by:

1. Moving the thread-pool to the FFI itself (sharing it between FVM
versions).
2. Setting the thread-pool size equal to `LOTUS_FVM_CONCURRENCY`.

It also defaults `LOTUS_FVM_CONCURRENCY` to the number of available
CPU threads instead of the previous 4.

NOTE: I've also tried increasing the stack size instead of using
threads, but Go _does not_ like it when other foreign mess with the
stack size of _its_ threads (but it has no problem if we create our own
threads).

fixes filecoin-project/lotus#11817

We previously used the FVM's `ThreadedExecutor` to execute messages on
separate threads because the FVM requires 64MiB of stack space.

1. The FVM v3 supported for 8 concurrent threads.
2. The FVM v4 supports up to the number of CPU threads available.

Unfortunately, neither version was influenced by the
`LOTUS_FVM_CONCURRENCY` environment variable.

This patch fixes this by:

1. Moving the thread-pool to the FFI itself (sharing it between FVM
versions).
2. Setting the thread-pool size equal to `LOTUS_FVM_CONCURRENCY`.

It also defaults `LOTUS_FVM_CONCURRENCY` to the number of available
CPU threads instead of the previous 4.

NOTE: I've also tried increasing the stack size instead of using
threads, but Go _does not_ like it when other foreign mess with the
stack size of _its_ threads (but it has no problem if we create our own
threads).

fixes filecoin-project/lotus#11817